Infectious Disease and Microbiology – Psychrobacter immobilis

Overview

Psychrobacter immobilis is a Gram-negative coccobacillus and an uncommon opportunistic pathogen. Members of the genus Psychrobacter are notable for their ability to survive and grow in cold environments, and organisms within this group have been isolated from food, environmental sources, and clinical specimens.

Human disease caused by P. immobilis is rare, with only a limited number of documented infections. The organism has been recovered from blood, urine, and wounds, but because clinical experience is limited, treatment should be guided by culture and antimicrobial susceptibility testing rather than a standard empiric regimen.

Classification

Genus: Psychrobacter

Species: Psychrobacter immobilis

Organism type: Gram-negative coccobacillus

The taxonomy and identification of some organisms historically assigned to Psychrobacter or closely related genera have undergone revision, so older reports may contain uncertain or outdated species assignments.

Microbiologic Characteristics

Psychrobacter organisms are generally:

• Gram-negative coccobacilli

• Aerobic

• Nonmotile

• Non-spore-forming

• Usually oxidase positive

• Usually catalase positive

• Capable of surviving or growing at relatively low temperatures

Their short, plump morphology can sometimes make them appear almost coccal on Gram stain.

High-Yield Microbiology Pattern

Gram-negative coccobacillus

• ●

Cold-tolerant environmental organism

• ●

Rare opportunistic infection

→ Consider Psychrobacter

Psychrotolerance

The name:

Psychrobacter

reflects the organism’s association with:

Cold environments

Members of the genus can tolerate and, depending on the species and conditions, grow at temperatures lower than those tolerated by many conventional human pathogens.

This explains their recovery from:

• Refrigerated foods

• Cold environmental habitats

• Marine environments

• Other low-temperature ecological niches

Incubation Period

The incubation period is:

Unknown

Because P. immobilis is a rare opportunistic pathogen rather than an organism producing a well-defined transmissible syndrome, there is no characteristic incubation interval.

Epidemiology

P. immobilis has been isolated from:

Food

and is associated with:

Cold environments

Psychrobacter organisms are broadly environmental and have been recovered from a variety of ecological sources.

Human infection remains:

Uncommon

Clinical Significance

An important principle when Psychrobacter is isolated from a clinical specimen is determining whether the isolate represents:

True infection

or

Colonization/contamination

This distinction depends on the:

• Clinical syndrome

• Specimen source

• Host factors

• Reproducibility of the culture

• Presence of systemic inflammatory findings

Bloodstream Infection

P. immobilis has been isolated from:

Blood

and rare cases of clinically significant:

Bacteremia

have been reported.

Isolation from multiple blood cultures in a patient with compatible signs of infection provides stronger evidence for true invasive disease.

High-Yield Blood Culture Pattern

Unusual Gram-negative coccobacillus

• ●

Repeated positive blood cultures

• ●

Compatible systemic illness

→ Consider true Psychrobacter bacteremia rather than automatically dismissing the isolate

Urinary Isolation

The organism has also been recovered from:

Urine specimens

Clinical significance should be assessed using the same principles applied to other unusual urinary isolates.

Evidence supporting true UTI includes:

• Urinary symptoms

• Pyuria

• Significant bacterial growth

• Repeated isolation

• Appropriate host risk factors

Wound Infection

P. immobilis has been isolated from:

Wound specimens

Because wounds can contain colonizing environmental organisms, recovery from a superficial specimen does not necessarily prove invasive infection.

True wound infection becomes more likely when there is:

Clinical inflammation

• ●

Deep-tissue isolation

• ●

Compatible microbiologic findings

Other Psychrobacter Infections

Although the source specifically discusses P. immobilis, other members of the genus Psychrobacter have occasionally been implicated in human disease.

Reported infections within the genus have included:

• Bacteremia

• Wound and soft-tissue infections

• Urinary infection

• Other rare invasive infections

Overall, however:

Human disease is uncommon.

Diagnosis

The primary diagnostic method is:

Culture

Depending on the clinical presentation, specimens may include:

• Blood

• Urine

• Wound material

• Deep tissue

• Other normally sterile specimens

Laboratory Identification

Accurate species identification may be challenging because:

• Psychrobacter is uncommon in clinical laboratories

• Historical taxonomy has changed

• Phenotypically similar organisms may be confused with related Gram-negative bacteria

Modern laboratory identification methods can improve species-level classification.

Interpretation of Culture

Because the organism is rare, a positive culture should always be interpreted in:

Clinical context

Particularly convincing evidence of true infection includes:

Isolation from a normally sterile site

• ●

Repeated recovery

• ●

Compatible clinical disease

Treatment

There are:

Insufficient published clinical data

to establish a single standard treatment regimen specifically for P. immobilis infection.

Therefore, antimicrobial selection should be based primarily on:

In vitro antimicrobial susceptibility testing

Treatment Principle

Culture

↓

Confirm organism identification

↓

Perform susceptibility testing

↓

Select an active antimicrobial

↓

Assess for an underlying source

↓

Provide source control when necessary

Why Susceptibility Testing Matters

Because Psychrobacter infections are:

Rare

there are insufficient clinical data to confidently recommend one antimicrobial regimen for every case.

Therefore, therapy should account for:

• Susceptibility results

• Site of infection

• Severity

• Host immune status

• Renal and hepatic function

• Presence of a removable infectious source

Source Control

When infection involves a wound or another localized source, management may also require:

• Wound cleaning

• Drainage of infected collections

• Debridement when appropriate

• Removal of infected foreign material when relevant

Thus, management should not rely solely on antimicrobial therapy when a correctable source is present.

Psychrobacter vs. Acinetobacter

These organisms can have some superficial microbiologic similarities.

Psychrobacter

→ Gram-negative coccobacillus

→ Usually oxidase positive

→ Associated with cold environments

→ Rare human pathogen

Acinetobacter

→ Gram-negative coccobacillus

→ Oxidase negative

→ Important healthcare-associated pathogen

→ A. baumannii can exhibit extensive multidrug resistance

The:

Oxidase reaction

can therefore be a useful distinguishing clue.

Psychrobacter vs. Moraxella

Both may appear as:

Gram-negative coccoid or coccobacillary organisms

and both are often:

Oxidase positive

However, Psychrobacter is particularly associated with:

Environmental and cold-tolerant organisms

whereas Moraxella catarrhalis is a well-established human respiratory pathogen.

Prevention

There are no specific preventive measures or vaccines directed against P. immobilis because human disease is:

Extremely uncommon

General principles include:

• Appropriate wound hygiene

• Proper handling of invasive devices

• Standard infection-control practices

• Proper food handling

High-Yield Clinical Pattern

Cold-associated environmental organism

• ●

Gram-negative coccobacillus

• ●

Rare isolation from blood, urine, or wounds

→ Think Psychrobacter immobilis

Exam Essentials

Genus: Psychrobacter

Species: P. immobilis

Type: Gram-negative coccobacillus

Metabolism: Generally aerobic

Motility: Nonmotile

Oxidase: Usually positive

Catalase: Usually positive

Environmental feature: Cold tolerant/psychrotolerant

Reservoir: Environmental sources and food, particularly in colder settings

Incubation: Unknown

Human infection: Very rare

Clinical specimens: Blood, urine, and wounds

Possible disease: Bacteremia, UTI, and wound infection

Diagnosis: Culture

Diagnostic challenge: Uncommon organism with historically evolving taxonomy and possible identification difficulties

Standard treatment: Not established

Treatment principle: Antimicrobial susceptibility-guided therapy

Additional management: Source control when clinically indicated

Key clinical pearl: Psychrobacter immobilis is a rare, cold-tolerant Gram-negative coccobacillus associated primarily with environmental and food sources. Human disease is uncommon, but the organism has been recovered from blood, urine, and wounds. Because clinical treatment data are sparse and taxonomy has historically been difficult, a clinically significant isolate should be accurately identified and treated according to in vitro antimicrobial susceptibility results rather than a fixed empiric regimen.

Infectious Disease and Microbiology – Pseudomonas luteola

Overview

Pseudomonas luteola is a Gram-negative bacillus that is an uncommon cause of human infection. It was formerly known as Chryseomonas luteola and is encountered primarily as an opportunistic pathogen.

Human infections are rare but have been reported worldwide. Important clinical manifestations include bacteremia/septicemia, central venous catheter-associated infection, wound infection, peritoneal dialysis-associated peritonitis, prosthetic valve endocarditis, and meningitis.

Classification

Genus: Pseudomonas

Species: Pseudomonas luteola

Former name: Chryseomonas luteola

Organism type: Gram-negative bacillus

Microbiologic Characteristics

P. luteola is a:

• Gram-negative bacillus

• Environmental organism

• Opportunistic pathogen

• Rare cause of invasive human infection

An important historical laboratory characteristic is the production of:

Yellow-pigmented colonies

The species name luteola reflects this yellow pigmentation.

High-Yield Microbiology Pattern

Gram-negative bacillus

• ●

Yellow-pigmented colonies

• ●

Rare opportunistic infection

• ●

Foreign body or catheter

→ Consider Pseudomonas luteola

Epidemiology

P. luteola has a:

Worldwide distribution

However, human infection is:

Rare

The organism is primarily environmental and may be encountered in:

• Soil

• Water

• Moist environmental sources

Human disease generally occurs opportunistically rather than as a common community infection.

Risk Factors

Reported infections frequently occur in patients with:

• Central venous catheters

• Prosthetic cardiac material

• Peritoneal dialysis catheters

• Recent surgery or trauma

• Open wounds

• Other invasive medical devices

• Significant underlying disease

Thus, the presence of:

Foreign material

is an important clinical clue.

Septicemia and Bacteremia

One of the important manifestations is:

Septicemia/bacteremia

Some cases may be:

Polymicrobial

The source particularly emphasizes an association with:

Central venous catheters

Central Venous Catheter Infection

A central venous catheter can provide a surface for:

Bacterial adherence

and potentially persistent bloodstream infection.

A typical sequence is:

Environmental organism

↓

Contamination or colonization of device

↓

Catheter-associated infection

↓

Bacteremia/septicemia

High-Yield Clinical Pattern

Central venous catheter

• ●

Unusual Gram-negative bacillus

• ●

Bacteremia

→ Consider an opportunistic environmental organism such as P. luteola

Polymicrobial Infection

The source notes that septicemia may sometimes be:

Polymicrobial

Therefore, isolation of P. luteola does not necessarily mean that it is the only organism contributing to the infection.

Management should address:

All clinically significant pathogens

identified from appropriate cultures.

Wound Infection

P. luteola can cause:

Wound infections

This is consistent with its environmental reservoir and opportunistic behavior.

Potential settings include:

• Traumatic wounds

• Surgical wounds

• Contaminated tissue

• Patients with impaired host defenses

Peritoneal Dialysis-Associated Peritonitis

An important manifestation is:

Peritonitis

in patients receiving:

Peritoneal dialysis

The dialysis catheter provides a potential route by which environmental organisms can enter the peritoneal cavity.

High-Yield Dialysis Pattern

Peritoneal dialysis

• ●

Peritonitis

• ●

Rare environmental Gram-negative bacillus

→ Consider Pseudomonas luteola

Prosthetic Valve Endocarditis

P. luteola has also been reported as a cause of:

Prosthetic valve endocarditis

The presence of:

Prosthetic cardiac material

provides a surface that may facilitate persistent infection.

High-Yield Endocarditis Pattern

Prosthetic heart valve

• ●

Endocarditis

• ●

Unusual environmental Gram-negative bacillus

→ Consider P. luteola

Meningitis

Rare cases of:

Meningitis

have also been reported.

This represents an uncommon but potentially serious invasive manifestation.

Diagnosis

The primary diagnostic method is:

Culture

Depending on the clinical syndrome, appropriate specimens may include:

• Blood

• Catheter-associated blood cultures

• Wound specimens

• Peritoneal dialysis fluid

• Cerebrospinal fluid

• Other normally sterile specimens

Organism Identification

Because P. luteola is uncommon, accurate laboratory identification is important.

The historical name:

Chryseomonas luteola

may appear in older literature.

Recognition of the organism as a potential pathogen is especially important when it is repeatedly recovered from:

Normally sterile sites

or in association with:

Intravascular/prosthetic devices

Contamination vs. True Infection

Because P. luteola is an environmental organism, an unusual culture result should be interpreted within the clinical context.

True infection is more likely when there is:

• Compatible clinical disease

• Repeated positive cultures

• Isolation from a normally sterile site

• Central venous catheter involvement

• Prosthetic material

• Peritoneal dialysis-associated infection

• Appropriate inflammatory findings

Treatment

The source lists:

Ceftazidime

as the principal treatment.

However, because P. luteola infections are uncommon and susceptibility patterns can vary, treatment should ideally be guided by:

Culture and antimicrobial susceptibility testing

Additional Treatment Options

The source lists:

• Ureidopenicillins

• Aminoglycosides

• Imipenem

• Meropenem

• Ciprofloxacin

The appropriate antimicrobial depends on:

• Susceptibility results

• Site of infection

• Severity

• Renal function

• Presence of prosthetic or catheter material

Source Control

Because many reported infections involve:

Foreign material

management may require more than antimicrobial therapy.

Possible interventions include:

• Removal or replacement of an infected central venous catheter

• Management of an infected peritoneal dialysis catheter

• Debridement of an infected wound

• Evaluation of infected prosthetic cardiac material

High-Yield Treatment Principle

Device-associated P. luteola infection

→ Culture and susceptibility-guided antibiotics

• ●

→ Appropriate source control

Pseudomonas luteola vs. Pseudomonas aeruginosa

Although both carry the Pseudomonas name, they should not automatically be considered clinically identical.

Pseudomonas aeruginosa

→ Major human opportunistic pathogen

→ Common healthcare-associated infections

→ Characteristic blue-green pigments

→ Numerous well-established virulence factors

→ Frequently multidrug resistant

Pseudomonas luteola

→ Rare human pathogen

→ Environmental organism

→ Historically called Chryseomonas luteola

→ Characteristically associated with yellow pigmentation

→ Reported particularly with catheters and prosthetic material

Device-Associated Disease

A useful way to remember P. luteola is:

RARE ENVIRONMENTAL GRAM-NEGATIVE BACILLUS

• ●

FOREIGN BODY

The important foreign-body settings include:

Central venous catheter

Peritoneal dialysis catheter

and

Prosthetic heart valve

High-Yield Clinical Pattern

Rare environmental Gram-negative bacillus

• ●

Yellow-pigmented colonies

• ●

Central venous catheter-associated bacteremia

→ Think Pseudomonas luteola

Exam Essentials

Genus: Pseudomonas

Species: P. luteola

Former name: Chryseomonas luteola

Type: Gram-negative bacillus

Distribution: Worldwide

Frequency: Rare human infection

Reservoir: Primarily environmental

Pigment: Characteristically yellow

Clinical behavior: Opportunistic pathogen

Major infection: Bacteremia/septicemia

Important association: Central venous catheters

Other infections: Wound infection, peritoneal dialysis-associated peritonitis, prosthetic valve endocarditis, and meningitis

Polymicrobial disease: May occur

Diagnosis: Culture

Source treatment: Ceftazidime

Additional source agents: Ureidopenicillins, aminoglycosides, imipenem, meropenem, and ciprofloxacin

Treatment principle: Susceptibility-guided antimicrobial therapy

Device-associated infection: Consider appropriate source control

Key clinical pearl: Pseudomonas luteola, formerly Chryseomonas luteola, is a rare environmental Gram-negative bacillus notable for yellow pigmentation and opportunistic infections involving foreign material. The most useful clinical clue is an unusual Gram-negative bacteremia associated with a central venous catheter, although peritoneal dialysis peritonitis and prosthetic valve endocarditis are also characteristic reported presentations.

Infectious Disease and Microbiology – Pseudallescheria boydii

Overview

Pseudallescheria boydii is a filamentous fungus (mold) characterized by septate, hyaline hyphae. It has historically also been associated with the name Scedosporium apiospermum, although modern taxonomy distinguishes sexual and asexual forms within the Scedosporium/Pseudallescheria complex.

This environmental mold has a worldwide distribution and can cause both localized and invasive disease. Important manifestations include eumycetoma, pulmonary infection, brain abscess, osteomyelitis, sinusitis, ocular infection, endocarditis, fungal balls, and disseminated infection.

A particularly important clinical feature is that its invasive infections can closely resemble aspergillosis, but P. boydii/Scedosporium is characteristically poorly susceptible or resistant to amphotericin B.

Classification

Genus: Pseudallescheria

Species: Pseudallescheria boydii

Historical/related terminology:

Scedosporium apiospermum

Organism type: Filamentous fungus (mold)

Hyphae: Hyaline and septate

Microbiologic Characteristics

The characteristic morphology includes:

• Septate hyaline hyphae

• Filamentous mold growth

• Fungal aggregates or granules in mycetoma

• Hyaline septate hyphae in invasive hyalohyphomycosis

Because the hyphae may resemble those of Aspergillus, microbiologic identification is important.

High-Yield Microbiology Pattern

Hyaline mold

• ●

Septate hyphae

• ●

Aspergillus-like appearance

• ●

Poor amphotericin B activity

→ Think Pseudallescheria/Scedosporium

Incubation Period

The incubation period is:

Unknown

For mycetoma, infection is generally chronic and slowly progressive following environmental inoculation.

Epidemiology

P. boydii has a:

Worldwide distribution

The organism occurs environmentally in:

• Soil

• Polluted water

• Sewage-contaminated environments

• Organic material

Mycetoma is particularly common in:

Tropical and subtropical regions

Transmission

Localized infection may develop after:

Traumatic inoculation

of contaminated environmental material into:

Skin or subcutaneous tissue

This helps explain the frequent involvement of exposed areas such as:

Feet and hands

Mycetoma

One of the classic manifestations of P. boydii infection is:

MYCETOMA

Mycetoma is a chronic infection involving:

Skin + subcutaneous tissue

and may eventually extend into:

Bone

Classic Mycetoma Triad

The classic clinical triad is:

Tumefaction

• ●

Draining sinus tracts

• ●

Granules in the drainage

→ MYCETOMA

This is one of the most important exam patterns associated with the disease.

Tumefaction

Mycetoma produces a:

Slowly enlarging, tumor-like swelling

The lesion is usually chronic and may progress over a prolonged period.

Draining Sinus Tracts

As infection progresses, multiple:

Sinus tracts

may develop between the infected tissue and skin surface.

These tracts can discharge:

Pus containing characteristic granules

Granules

The granules contain aggregates of the causative organism.

In fungal mycetoma, microscopic examination may demonstrate:

Fungal hyphae within the granules

Recognition and culture of these structures can help identify the causative organism.

High-Yield Mycetoma Pattern

Tropical exposure

• ●

Chronic swelling of foot

• ●

Multiple draining sinuses

• ●

Granules in pus

→ Think MYCETOMA

Eumycetoma vs. Actinomycetoma

Mycetoma can be caused by either:

True fungi

or

Filamentous bacteria

Eumycetoma

Caused by:

Fungi

Examples include:

Pseudallescheria/Scedosporium

and other mycetoma-producing molds.

Actinomycetoma

Caused by filamentous bacteria such as:

Nocardia

and certain other aerobic actinomycetes.

Historically, Actinomyces has also appeared in discussions of mycetoma-like infections.

High-Yield Distinction

Eumycetoma

→ Fungal

Actinomycetoma

→ Bacterial

Both may produce:

Swelling + sinus tracts + granules

Sites of Mycetoma

The most characteristic sites are:

Feet

and

Hands

because these exposed areas are susceptible to traumatic environmental inoculation.

However, infection may occur on:

Any exposed body surface

Madura Foot

Mycetoma involving the foot is classically known as:

Madura foot

The clinical picture is:

Chronic foot swelling

• ●

Draining sinus tracts

• ●

Granules

Osteomyelitis

Chronic infection may extend from soft tissue into:

Underlying bone

resulting in:

Osteomyelitis

Bone involvement is an important complication of advanced mycetoma.

Hyalohyphomycosis

Outside the classic mycetoma syndrome, P. boydii can produce:

Hyalohyphomycosis

This refers to infection caused by molds that demonstrate:

Hyaline, septate hyphae in tissue

Aspergillus-Like Disease

An important diagnostic problem is that invasive P. boydii infection can resemble:

Aspergillus

in terms of:

• Clinical presentation

• Tissue appearance

• Septate hyphal morphology

Therefore:

Septate hyphae in tissue do not automatically mean Aspergillus.

Culture or other organism-specific identification is important.

High-Yield Diagnostic Trap

Septate hyaline hyphae

→ Do not automatically diagnose Aspergillus

If culture identifies:

Pseudallescheria/Scedosporium

the therapeutic implications are important because of differing antifungal susceptibility.

Pulmonary Infection

P. boydii may cause:

Pneumonia

particularly in susceptible patients.

Pulmonary disease can resemble:

Invasive pulmonary aspergillosis

and may occur in patients with underlying lung disease or impaired immunity.

Fungal Ball

The organism can colonize preexisting pulmonary cavities and form:

Fungal balls

This can closely resemble an:

Aspergilloma

High-Yield Pulmonary Pattern

Preexisting lung cavity

• ●

Fungal ball

• ●

Aspergillus-like septate hyphae

→ Consider Scedosporium/Pseudallescheria as well as Aspergillus

Brain Abscess

A particularly serious manifestation is:

Brain abscess

CNS disease can occur following dissemination or particular environmental exposures.

Neurologic manifestations depend on the location and extent of infection.

Near-Drowning Association

An especially important clinical association with Scedosporium apiospermum is:

Near-drowning in contaminated water

followed later by:

Central nervous system infection or brain abscess

This organism can be present in polluted water, making this exposure an important diagnostic clue.

High-Yield CNS Pattern

Near-drowning

• ●

Contaminated/polluted water exposure

• ●

Delayed brain abscess

→ Think Scedosporium apiospermum

Meningitis

The source also identifies:

Meningitis

as a possible manifestation.

CNS infection is serious and may be difficult to treat.

Eye Infections

Ocular manifestations may include:

• Keratitis

• Endophthalmitis

• Other invasive ocular infections

These may follow:

Trauma

or other direct inoculation events.

Sinusitis

P. boydii may cause:

Fungal sinusitis

The clinical and histopathologic appearance may resemble infection caused by other hyaline molds.

Endocarditis

Rarely, the organism may cause:

Endocarditis

This is a serious invasive manifestation and may require combined:

Antifungal therapy + surgical management

depending on the circumstances.

Disseminated Infection

In susceptible patients, P. boydii can cause:

Disseminated fungal infection

with involvement of multiple organs.

Risk is increased in patients with:

Significant immunosuppression

Diagnosis

Diagnosis is based on:

Culture

and

Identification of the organism in tissue biopsy specimens

For mycetoma, examination of:

Granules from draining sinus tracts

is particularly useful.

Examination of Granules

The granular material discharged from mycetoma lesions contains:

Aggregates of fungal elements

Microscopic examination can provide clues to the causative organism.

Culture is needed for more specific identification.

Histopathology

Tissue examination may demonstrate:

Septate hyaline hyphae

However, this appearance can resemble:

Aspergillus

Therefore, morphology alone may not reliably distinguish the two.

Culture

Culture is particularly important because:

Correct identification directly affects treatment

This is especially true when differentiating Scedosporium/Pseudallescheria from Aspergillus.

Treatment

The source emphasizes that treatment data are limited.

Historically, high-dose azoles such as:

• Itraconazole

• Miconazole

• Ketoconazole

were recommended.

For invasive Scedosporium apiospermum/Pseudallescheria boydii infection, voriconazole has become an especially important systemic antifungal option.

Amphotericin B Resistance

One of the most important treatment facts is:

AMPHOTERICIN B IS OFTEN INEFFECTIVE

against P. boydii/S. apiospermum.

This contrasts with many other serious invasive mold infections.

High-Yield Treatment Pattern

Aspergillus-like invasive mold infection

• ●

Septate hyaline hyphae

• ●

Poor response/resistance to amphotericin B

→ Think Scedosporium/Pseudallescheria

Surgical Management

Localized infection may require:

Surgical excision or debridement

This is particularly important for:

• Mycetoma

• Osteomyelitis

• Localized deep infection

• Abscesses

• Infected or necrotic tissue

Thus, management may require:

ANTIFUNGAL THERAPY

• ●

SURGICAL SOURCE CONTROL

Treatment of Mycetoma

Fungal mycetoma can be difficult to eradicate.

Management may require:

• Prolonged antifungal therapy

• Surgical debridement

• Excision of localized lesions

• Management of associated osteomyelitis

Extensive disease may be particularly challenging.

Pseudallescheria/Scedosporium vs. Aspergillus

Both can demonstrate:

Hyaline septate hyphae

and both may cause:

• Pneumonia

• Sinusitis

• CNS infection

• Fungal balls

• Disseminated disease

However:

Aspergillus

→ Classic acute-angle branching septate hyphae

→ Amphotericin B formulations can have activity against some species

Scedosporium apiospermum / Pseudallescheria boydii

→ Can closely mimic Aspergillus

→ Amphotericin B often has poor activity

→ Voriconazole is an important therapeutic agent

→ Notable association with CNS infection after near-drowning

Pseudallescheria vs. Mucorales

Pseudallescheria/Scedosporium

→ Septate, hyaline hyphae

Mucorales

→ Broad, ribbon-like, typically pauciseptate or aseptate hyphae

Thus, the hyphal morphology can help narrow the differential.

Pseudallescheria vs. Nocardia

Both may be associated with:

Mycetoma

However:

Pseudallescheria

→ Fungus

→ Hyaline septate hyphae

→ Eumycetoma

Nocardia

→ Filamentous bacterium

→ Gram-positive branching organism

→ Often weakly acid-fast

→ Actinomycetoma

Prevention

There is no specific vaccine.

General preventive measures include:

• Protecting feet and hands from penetrating environmental trauma

• Wearing footwear in endemic areas

• Cleaning contaminated wounds

• Appropriate management of traumatic injuries

• Avoiding unnecessary exposure of open wounds to contaminated soil or water

High-Yield Mycetoma Pattern

Tropical region

• ●

Chronic painless swelling of foot

• ●

Draining sinus tracts

• ●

Granules

→ Mycetoma

If caused by a hyaline mold such as Pseudallescheria:

→ Eumycetoma

High-Yield Invasive Pattern

Near-drowning in polluted water

• ●

Delayed CNS infection/brain abscess

• ●

Septate hyaline mold

→ Think Scedosporium apiospermum

Exam Essentials

Historical name: Pseudallescheria boydii

Related/currently used name: Scedosporium apiospermum complex

Type: Filamentous fungus (mold)

Hyphae: Hyaline and septate

Distribution: Worldwide

Mycetoma distribution: More common in tropical/subtropical regions

Classic localized disease: Eumycetoma

Classic mycetoma triad: Tumefaction + draining sinuses + granules

Classic site: Foot (Madura foot) and hands

Possible complication: Osteomyelitis

Other infections: Pneumonia, sinusitis, brain abscess, meningitis, ocular infection, endocarditis, fungal balls, and disseminated infection

Important exposure: Near-drowning → CNS infection/brain abscess

Histopathology: Septate hyaline hyphae

Major mimic: Aspergillus

Diagnosis: Culture + tissue biopsy, with examination of mycetoma granules when present

Major treatment clue: Amphotericin B often ineffective

Important systemic agent: Voriconazole

Source-listed azoles: High-dose itraconazole, miconazole, or ketoconazole

Source control: Surgical debridement/excision may be necessary

Key clinical pearl: Pseudallescheria boydii/Scedosporium apiospermum is a hyaline, septate mold that can cause eumycetoma or invasive disease closely resembling aspergillosis. Remember two particularly high-yield clues: chronic swelling with draining sinuses and granules indicates mycetoma, while delayed brain abscess after near-drowning in polluted water strongly suggests Scedosporium. Unlike many invasive molds, it often responds poorly to amphotericin B, making correct identification therapeutically important.

Infectious Disease and Microbiology – Providencia Species

Overview

Providencia species are Gram-negative bacilli within the order Enterobacterales. They are opportunistic pathogens particularly associated with healthcare-associated infections, especially complicated and catheter-associated urinary tract infections.

Important clinical manifestations include UTI, intra-abdominal infection, central venous catheter-related infection, and bacteremia. Providencia can develop substantial antimicrobial resistance, including extended-spectrum β-lactamase (ESBL) and other resistance mechanisms, making culture and susceptibility testing particularly important.

Classification

Genus: Providencia

Important species include:

• Providencia alcalifaciens

• Providencia rettgeri

• Providencia rustigianii

• Providencia stuartii

• Other species

Among these, P. stuartii and P. rettgeri are particularly important in healthcare-associated urinary infections.

Microbiologic Characteristics

Providencia species are generally:

• Gram-negative bacilli

• Facultatively anaerobic

• Members of Enterobacterales

• Motile

• Oxidase negative

• Phenylalanine deaminase positive

• Generally non-lactose fermenting

They are microbiologically related to:

Proteus

and

Morganella

High-Yield Microbiology Pattern

Gram-negative bacillus

• ●

Enterobacterales

• ●

Phenylalanine deaminase positive

• ●

Healthcare-associated UTI

→ Think Providencia

Incubation Period

A specific incubation period is usually not applicable because infection is commonly:

Endogenous or healthcare associated

Disease often develops in patients with underlying medical conditions or invasive devices.

Epidemiology

Providencia is particularly important as a:

Nosocomial pathogen

Risk factors include:

• Prolonged hospitalization

• Long-term care residence

• Chronic urinary catheterization

• Urinary tract abnormalities

• Repeated antimicrobial exposure

• Severe underlying disease

• Invasive medical devices

Providencia stuartii

Providencia stuartii is particularly associated with:

Long-term urinary catheterization

and

Healthcare-associated urinary tract infection

It has historically been an important pathogen in:

Long-term care facilities and chronically catheterized patients

High-Yield Clinical Association

Long-term care patient

• ●

Chronic urinary catheter

• ●

Gram-negative UTI

→ Consider Providencia stuartii

Providencia rettgeri

Providencia rettgeri is another clinically important species associated with:

• Urinary tract infections

• Healthcare-associated infections

• Bacteremia

Like other Providencia species, antimicrobial resistance may complicate treatment.

Urinary Tract Infection

The urinary tract is one of the most important sites of Providencia infection.

Clinical syndromes include:

• Catheter-associated UTI

• Complicated cystitis

• Pyelonephritis

• Urosepsis

The association with:

Indwelling urinary catheters

is particularly important.

Urease Production

Some clinically important Providencia species, particularly P. rettgeri, may produce:

UREASE

Urease breaks down urea and can contribute to:

Urinary alkalinization

and potentially:

Stone formation and catheter encrustation

This provides an important microbiologic connection with Proteus, although the classic urease–struvite association remains strongest with Proteus mirabilis.

Catheter-Associated UTI

Urinary catheters provide:

A surface for bacterial adherence and biofilm formation

and allow organisms to persist within the urinary tract.

Thus:

Chronic catheter

↓

Colonization/biofilm

↓

Persistent or recurrent bacteriuria

↓

Symptomatic UTI

↓

Possible ascending infection

↓

Bacteremia/urosepsis

High-Yield UTI Pattern

Hospitalized or long-term care patient

• ●

Chronic urinary catheter

• ●

Resistant Gram-negative bacillus

→ Think Providencia, particularly P. stuartii

Bacteremia

Providencia species can cause:

Bacteremia

The source may be:

• Urinary tract

• Central venous catheter

• Intra-abdominal infection

• Other invasive infection

Urinary-source bacteremia is particularly important in patients with complicated UTI.

Central Venous Catheter-Related Infection

The source also identifies:

Central venous catheter-related infection

as an important manifestation.

Management should include consideration of:

Catheter source control

in addition to appropriate antimicrobial therapy.

Intra-Abdominal Infection

Providencia may occasionally participate in:

Intra-abdominal infection

These infections can be:

Polymicrobial

and may require antimicrobial coverage beyond Providencia alone.

Abscess or other surgically correctable sources may require:

Drainage or operative source control

Nosocomial Infection

A major epidemiologic characteristic is the organism’s association with:

Healthcare-associated infection

This is clinically important because healthcare-associated isolates are more likely to have been exposed to antimicrobial selection pressure and may demonstrate:

Multidrug resistance

Diagnosis

The principal diagnostic method is:

Culture

Depending on the clinical syndrome, specimens may include:

• Urine

• Blood

• Central-line-associated blood cultures

• Abscess material

• Intra-abdominal specimens

Antimicrobial Susceptibility Testing

Susceptibility testing is particularly important because Providencia may possess or acquire multiple resistance mechanisms.

Therefore:

Identification alone is not sufficient

The isolate’s:

Antimicrobial susceptibility profile

should guide definitive treatment whenever possible.

Antimicrobial Resistance

Providencia species may demonstrate resistance involving:

• β-lactam antibiotics

• Fluoroquinolones

• Aminoglycosides

• Multiple other antimicrobial classes

Some strains can produce:

Extended-spectrum β-lactamases (ESBLs)

and more extensively resistant isolates may possess:

Carbapenemases

or other mechanisms.

ESBL

Extended-spectrum β-lactamases

can hydrolyze many:

Penicillins

and

Extended-spectrum cephalosporins

This is particularly important in healthcare-associated Gram-negative infections.

High-Yield Resistance Pattern

Nosocomial Providencia infection

• ●

Resistance to extended-spectrum cephalosporins

→ Consider ESBL or another β-lactam resistance mechanism

→ Use susceptibility results to guide therapy

Treatment

The source lists:

Meropenem

or

Imipenem

particularly when an ESBL-producing isolate is suspected.

It also lists:

Ciprofloxacin

as a treatment option.

Because susceptibility patterns vary considerably, definitive therapy should be based on:

Culture and antimicrobial susceptibility testing

rather than genus identification alone.

Additional Treatment Options

The source lists:

• Third-generation cephalosporins

• Aztreonam

• Ofloxacin

• Piperacillin–tazobactam

• Aminoglycosides

Whether any of these agents is appropriate depends on:

Documented susceptibility + infection severity + infection site

Important Aminoglycoside Consideration

An important microbiologic feature of Providencia is that susceptibility to individual aminoglycosides can be unpredictable.

In particular, some Providencia species have intrinsic resistance to certain aminoglycosides, so an aminoglycoside should not be assumed to be active without susceptibility information.

Source Control

Antimicrobial therapy alone may be insufficient when infected foreign material or an anatomic abnormality persists.

Important interventions may include:

• Removal or replacement of an unnecessary urinary catheter

• Management of urinary obstruction

• Evaluation of urinary stones

• Management of an infected central venous catheter

• Drainage of an intra-abdominal abscess

High-Yield Treatment Principle

Providencia infection

→ Obtain culture and susceptibility

• ●

Treat with an active antimicrobial

• ●

Correct the underlying catheter/device/anatomic source

Providencia vs. Proteus

Both belong to a related group of Enterobacterales and are:

Phenylalanine deaminase positive

Both may cause:

Urinary tract infection

However:

Proteus mirabilis

→ Famous for swarming motility

→ Strongly urease positive

→ Classic association with struvite/staghorn stones

Providencia

→ Particularly associated with nosocomial and catheter-associated UTI

→ P. stuartii is a classic long-term catheter organism

→ Antimicrobial resistance is an important concern

Providencia vs. Morganella

Providencia, Proteus, and Morganella form an important related group of Enterobacterales.

All may demonstrate:

Phenylalanine deaminase activity

and all can be opportunistic causes of:

Urinary tract and healthcare-associated infection

The specific organism is differentiated by biochemical testing and modern laboratory identification methods.

Providencia vs. E. coli

E. coli

→ Most common cause of community-acquired UTI

→ Common intestinal flora

→ Often lactose fermenting

Providencia

→ Less common overall

→ Particularly associated with healthcare settings

→ Important in chronically catheterized patients

→ Often non-lactose fermenting

→ Multidrug resistance can be prominent

Prevention

Prevention focuses primarily on reducing healthcare-associated infection.

Important measures include:

• Avoid unnecessary urinary catheterization

• Remove urinary catheters as soon as possible

• Maintain appropriate catheter care

• Use strict aseptic technique for central venous catheters

• Remove unnecessary vascular devices

• Follow infection-control precautions

• Use antimicrobials appropriately to reduce resistance selection

High-Yield Clinical Pattern

Long-term care or hospitalized patient

• ●

Chronic urinary catheter

• ●

Complicated UTI

• ●

Multidrug-resistant Gram-negative bacillus

→ Think Providencia, especially P. stuartii

Exam Essentials

Genus: Providencia

Important species: P. alcalifaciens, P. rettgeri, P. rustigianii, and P. stuartii

Type: Gram-negative bacillus

Order: Enterobacterales

Metabolism: Facultatively anaerobic

Oxidase: Negative

Motility: Generally motile

Phenylalanine deaminase: Positive

Related genera: Proteus and Morganella

Incubation: Usually not defined; often endogenous/healthcare associated

Major epidemiology: Nosocomial infection

Classic species: P. stuartii

Classic risk factor: Chronic urinary catheterization

Major infection: Complicated/catheter-associated UTI

Other infections: Intra-abdominal infection, central venous catheter infection, and bacteremia

Urease: Some species, particularly P. rettgeri, may be urease positive

Diagnosis: Culture

Resistance concern: Multidrug resistance and ESBL production; other resistance mechanisms may occur

Source ESBL treatment: Meropenem or imipenem

Source alternative: Ciprofloxacin if susceptible

Additional source agents: Third-generation cephalosporins, aztreonam, ofloxacin, piperacillin–tazobactam, and aminoglycosides

Management principle: Susceptibility-guided therapy + source control

Key clinical pearl: Providencia is a healthcare-associated Gram-negative bacillus that should be remembered particularly in chronically catheterized patients with complicated UTI. P. stuartii is the classic long-term catheter-associated species, while antimicrobial resistance—including ESBL and occasionally more extensive resistance mechanisms—makes culture and susceptibility-guided treatment essential.

Infectious Disease and Microbiology – Prototheca Species

Overview

Prototheca species are unusual unicellular, achlorophyllous algae that can cause a rare human infection known as protothecosis. Although they are algae, they lack chlorophyll and can grow on routine fungal media with a yeast-like appearance, which may lead to confusion with true fungi.

The most characteristic human manifestation is a chronic cutaneous or subcutaneous infection, often presenting as a relatively painless plaque or papulonodular lesion. Olecranon bursitis is another classic presentation. Disseminated disease, including bloodstream infection, peritonitis, and central nervous system infection, occurs mainly in immunocompromised patients.

Classification

Genus: Prototheca

Species listed in the source include:

• Prototheca stagnora

• Prototheca wickerhamii

• Prototheca zopfii

Organism type: Achlorophyllous alga

Disease: Protothecosis

Microbiologic Characteristics

Prototheca species are:

• Unicellular algae

• Achlorophyllous — they lack chlorophyll

• Capable of growing on fungal culture media

• Yeast-like in culture

• Reproduce by endosporulation

The lack of chlorophyll distinguishes Prototheca from typical photosynthetic algae.

High-Yield Microbiology Pattern

Unicellular alga

• ●

No chlorophyll

• ●

Yeast-like growth on fungal media

• ●

Endosporulation

→ Think Prototheca

Reproduction by Endosporulation

One of the most characteristic microbiologic features is:

Endosporulation

The parent cell, or:

Sporangium

develops multiple internal daughter cells known as:

Endospores

These are subsequently released and continue the reproductive cycle.

Morula-Like Appearance

On microscopic examination, the internal arrangement of endospores may produce a characteristic:

Morula-like appearance

or

“Soccer-ball” appearance

This can be a useful diagnostic clue in tissue or culture.

High-Yield Morphology Pattern

Large spherical cell

• ●

Multiple internal endospores

• ●

Morula/soccer-ball appearance

→ Prototheca

Epidemiology

Prototheca organisms are:

Ubiquitous in nature

They have been recovered particularly from:

• Soil

• Sewage

• Contaminated water

• Organic environmental material

Human protothecosis is:

Rare

but cases have been reported worldwide.

Transmission

Human infection is generally considered:

Environmental

rather than a person-to-person disease.

Cutaneous infection may follow environmental inoculation through:

Traumatized or damaged skin

Risk Factors

Although infection can occasionally occur in otherwise healthy individuals, more severe disease is associated with:

• Immunosuppression

• Malignancy

• Corticosteroid therapy

• Organ transplantation

• Other states of impaired host defense

Localized cutaneous infection and olecranon bursitis may occur without profound systemic immunosuppression.

Cutaneous Protothecosis

The best-described clinical syndrome is:

Skin and soft tissue infection

A characteristic lesion is:

Single + painless + plaque or papulonodular lesion

The lesion may subsequently:

Ulcerate

Clinical Appearance

Cutaneous disease may present with:

• Plaques

• Papules

• Nodules

• Papulonodular lesions

• Ulceration

• Chronic localized inflammation

The course is often:

Chronic and slowly progressive

rather than acutely toxic.

High-Yield Clinical Pattern

Environmental exposure

• ●

Chronic painless plaque/papulonodule

• ●

Occasional ulceration

• ●

Achlorophyllous algae in tissue

→ Think cutaneous protothecosis

Olecranon Bursitis

Another particularly characteristic manifestation is:

Olecranon bursitis

This involves inflammation and infection of the bursa overlying the:

Elbow

The association between Prototheca and chronic olecranon bursitis is sufficiently characteristic to be an important exam clue.

High-Yield Bursitis Pattern

Chronic olecranon bursitis

• ●

Environmental exposure or trauma

• ●

Unusual yeast-like organism

→ Consider Prototheca

Peritonitis

Rare cases of:

Peritonitis

have been reported in patients receiving:

Continuous ambulatory peritoneal dialysis (CAPD)

The peritoneal dialysis catheter provides an opportunity for an environmental organism to enter the peritoneal cavity.

High-Yield Dialysis Pattern

Peritoneal dialysis

• ●

Unusual persistent peritonitis

• ●

Yeast-like organism on fungal culture

→ Consider Prototheca

Meningitis

The source reports rare cases of:

Meningitis

particularly in patients with:

Advanced immunosuppression, including historically patients with AIDS.

Central nervous system disease is an uncommon manifestation of disseminated protothecosis.

Algaemia

Bloodstream infection with Prototheca has been described and may be called:

Algaemia

This manifestation is particularly associated with:

Immunocompromised hosts

and may indicate disseminated infection.

High-Yield Invasive Disease Pattern

Immunocompromised patient

• ●

Unusual yeast-like organism in blood

• ●

Organism actually identified as an achlorophyllous alga

→ Think Prototheca

Diagnosis

The principal diagnostic methods are:

Culture

and

Histopathologic examination of affected tissue

Because Prototheca grows on fungal media, it may initially resemble a yeast.

Culture

Prototheca species can grow on:

Routine fungal culture media

This characteristic contributes to their potential confusion with fungal organisms.

Species identification may require further microbiologic evaluation.

Histopathology

Biopsy of a cutaneous lesion may demonstrate:

Characteristic spherical organisms containing internal endospores

The presence of:

Multiple endospores within a sporangium

is an important morphologic clue.

Diagnostic Pattern

Chronic cutaneous lesion

• ●

Biopsy

• ●

Spherical sporangia containing multiple endospores

→ Prototheca

Differential Diagnosis

Because Prototheca can resemble yeast in culture or tissue, the differential may include:

• Fungal infection

• Chronic bacterial infection

• Other environmental organisms

• Noninfectious inflammatory lesions

Recognition of the characteristic:

Endosporulating sporangia

helps establish the diagnosis.

Prototheca vs. Yeast

Prototheca

→ Alga

→ Lacks chlorophyll

→ Yeast-like growth

→ Endosporulation

→ Sporangia containing multiple daughter cells

True yeast

→ Fungus

→ Commonly reproduces through budding or fission depending on species

→ Does not have the characteristic Prototheca endosporulating morphology

Prototheca vs. Chlorella

Both are algae, but:

Prototheca

→ Lacks chlorophyll

→ Nonphotosynthetic

→ Can cause human infection

Chlorella

→ Contains chlorophyll

→ Photosynthetic

Thus:

Prototheca = achlorophyllous alga

is the key distinction.

Treatment

The source lists:

Amphotericin B

as the principal treatment for protothecosis.

This is particularly important for:

Severe or disseminated disease

Additional Treatment

The source lists several azole antifungal agents:

• Itraconazole

• Ketoconazole

• Posaconazole

• Voriconazole

Despite Prototheca being an alga rather than a fungus, several conventional antifungal agents can have activity against it.

Treatment Considerations

Treatment should be individualized according to:

• Site of infection

• Severity

• Species

• Host immune status

• Susceptibility when available

Because protothecosis is rare, clinical experience is considerably more limited than for common fungal infections.

Surgical Management

Localized disease may sometimes benefit from:

Surgical excision or debridement

This can be particularly relevant for:

• Localized cutaneous lesions

• Olecranon bursitis

Thus, treatment may involve:

ANTIMICROBIAL THERAPY

• ●

SURGICAL SOURCE CONTROL

when appropriate.

Prevention

There is no specific vaccine or chemoprophylaxis.

General preventive measures include:

• Appropriate wound care

• Avoiding contamination of damaged skin

• Proper care of peritoneal dialysis equipment

• Appropriate catheter hygiene

• Infection-control practices in immunocompromised patients

High-Yield Clinical Pattern

Chronic painless skin plaque/nodule

or

Olecranon bursitis

• ●

Environmental exposure

• ●

Yeast-like growth on fungal media

• ●

Morula-like sporangia containing endospores

→ Think Prototheca

Exam Essentials

Genus: Prototheca

Important species: P. wickerhamii, P. zopfii, and P. stagnora

Organism: Unicellular achlorophyllous alga

Chlorophyll: Absent

Culture: Grows on fungal media with a yeast-like appearance

Reproduction: Endosporulation

Characteristic morphology: Sporangia containing multiple endospores

Classic appearance: Morula/soccer-ball pattern

Reservoir: Soil, sewage, water, and environmental organic material

Distribution: Worldwide

Disease: Protothecosis

Most characteristic infection: Cutaneous/soft tissue disease

Classic skin lesion: Painless plaque or papulonodular lesion, sometimes ulcerated

Classic focal association: Olecranon bursitis

Dialysis association: Peritonitis in peritoneal dialysis patients

Immunocompromised hosts: May develop disseminated infection, meningitis, or algaemia

Diagnosis: Culture + histopathology

Source treatment: Amphotericin B

Additional source treatments: Itraconazole, ketoconazole, posaconazole, or voriconazole

Localized disease: Surgical excision/debridement may sometimes be useful

Key clinical pearl: Prototheca is not a true fungus—it is an achlorophyllous alga that nevertheless grows with a yeast-like appearance on fungal media. The classic diagnostic combination is a chronic painless cutaneous plaque or olecranon bursitis with characteristic spherical sporangia containing multiple internal endospores, producing a morula or “soccer-ball” appearance.

Infectious Disease and Microbiology – Propionibacterium Species

Overview

Propionibacterium species are anaerobic Gram-positive bacilli that normally colonize human skin. The most clinically important species historically known as Propionibacterium acnes is now classified as Cutibacterium acnes.

These organisms are best known for their association with acne vulgaris and with indolent infections involving implanted or prosthetic material, particularly shoulder prostheses, cerebrospinal fluid shunts, prosthetic heart valves, and neurosurgical devices.

Classification

Historical genus: Propionibacterium

Important historical species: Propionibacterium acnes

Current terminology:

Propionibacterium acnes

→

Cutibacterium acnes

Organism type: Gram-positive bacillus

Taxonomy Update

The source lists:

P. ances

This appears to refer to:

P. acnes

The organism has subsequently been reclassified as:

Cutibacterium acnes

Therefore, the important modern name to remember is:

Cutibacterium acnes

Microbiologic Characteristics

C. acnes is generally:

• Gram-positive

• Bacillary or pleomorphic in morphology

• Anaerobic to aerotolerant

• Slow growing

• Non-spore-forming

• Part of normal skin flora

Its slow growth and normal presence on skin create an important diagnostic challenge:

Contaminant or true pathogen?

High-Yield Microbiology Pattern

Slow-growing Gram-positive bacillus

• ●

Normal skin flora

• ●

Prosthetic material

• ●

Indolent infection

→ Think Cutibacterium acnes

Incubation Period

The incubation period is:

Unknown

Most infections are:

Endogenous

and originate from the patient’s own skin microbiota.

This is especially important when organisms are introduced into deeper tissues during:

Surgery or implantation of prosthetic material

Epidemiology

Cutibacterium species are common components of:

Normal human skin flora

C. acnes particularly inhabits:

Sebaceous follicles

which helps explain its association with acne.

Pathogenesis

Two major clinical patterns should be remembered:

Sebaceous follicle colonization

→ Inflammation

→ Acne

and

Skin flora introduced during surgery

→ Adherence to implanted material

→ Biofilm formation

→ Chronic prosthetic/device infection

Biofilm Formation

An important characteristic of C. acnes is its ability to form:

Biofilms

on foreign material.

Biofilms allow organisms to:

• Adhere to prosthetic surfaces

• Persist despite host defenses

• Produce slowly progressive infection

• Become difficult to eradicate with antibiotics alone

This explains the organism’s strong association with:

Prosthetic and implanted-device infections

Acne Vulgaris

C. acnes is strongly associated with:

Acne vulgaris

The organism colonizes sebaceous follicles and participates in the inflammatory process associated with acne.

However, acne is multifactorial, involving more than bacterial colonization alone.

Important factors include:

• Follicular obstruction

• Sebum production

• Hormonal influences

• C. acnes

• Host inflammatory responses

High-Yield Acne Pattern

Sebaceous follicle

• ●

Comedones/inflammatory acne

• ●

Cutibacterium acnes

→ Acne vulgaris

Prosthetic Joint Infection

A major invasive manifestation is:

Prosthetic joint infection

C. acnes is especially important in:

Shoulder arthroplasty

and other orthopedic implants.

These infections may be relatively:

Indolent

rather than presenting with dramatic acute sepsis.

Shoulder Association

One of the strongest exam associations is:

Shoulder prosthesis

• ●

Slow, indolent postoperative infection

• ●

Slow-growing Gram-positive bacillus

→ Think Cutibacterium acnes

Clinical Presentation of Prosthetic Joint Infection

Possible manifestations include:

• Persistent joint pain

• Stiffness

• Reduced function

• Prosthetic loosening

• Subtle inflammatory findings

Fever and marked systemic illness may be absent.

This relatively subtle presentation can delay diagnosis.

CSF Shunt Infection

C. acnes can cause infections involving:

Cerebrospinal fluid shunts

The organism may adhere to the implanted device and produce:

Chronic or indolent shunt infection

Post-Neurosurgical Meningitis

The source also describes:

Meningitis following neurosurgical procedures

particularly in association with:

• Neurosurgical devices

• CSF shunts

• Other implanted material

Because the organism is slow growing, cultures may require appropriate incubation.

Endocarditis

C. acnes may cause:

Endocarditis

particularly when:

Prosthetic cardiac material

is present.

Prosthetic valve endocarditis may have a relatively indolent clinical course.

High-Yield Endocarditis Pattern

Prosthetic valve

• ●

Indolent endocarditis

• ●

Slow-growing skin-flora Gram-positive bacillus

→ Consider Cutibacterium acnes

Brain Abscess

The source also reports:

Brain abscess

as a possible manifestation, especially in clinical settings involving neurosurgery or implanted material.

Management may require:

Antimicrobial therapy

plus

Surgical drainage/source control

depending on the clinical situation.

Bacteremia

C. acnes can be recovered from:

Blood cultures

However, interpretation requires caution because the organism is:

Normal skin flora

and can contaminate blood cultures during collection.

Contaminant vs. True Pathogen

This is one of the most important clinical principles for C. acnes.

A positive culture may represent:

CONTAMINATION

or

TRUE INFECTION

Findings Supporting True Infection

True infection becomes more likely with:

• Multiple positive cultures

• Repeated isolation of the same organism

• Isolation from deep tissue surrounding a prosthesis

• Compatible clinical findings

• Prosthetic or implanted material

• Evidence of hardware loosening or infection

• Recovery from a normally sterile operative specimen

High-Yield Diagnostic Pattern

Cutibacterium in one blood culture

→ Could represent skin contamination

but

Repeated positive deep cultures + prosthetic material + compatible symptoms

→ Strongly consider true infection

Diagnosis

The principal diagnostic method is:

Culture

Because the organism grows relatively slowly and prefers anaerobic conditions, appropriate culture techniques are important.

Anaerobic Culture

Specimens should be obtained from:

• Deep tissue

• Prosthetic material

• Joint specimens

• CSF

• Blood

• Abscess material

Appropriate:

Anaerobic culture

and sufficient incubation time improve detection.

Diagnostic Challenge

C. acnes may be overlooked because:

It grows slowly

and

It is frequently assumed to be a contaminant

Therefore, clinical context is essential when determining whether a positive culture is significant.

Treatment of Acne

The source lists:

Tetracycline

or

Macrolide

therapy.

For acne, antimicrobial therapy is only one part of treatment, and modern management depends on acne severity and generally aims to limit unnecessary antibiotic exposure and resistance.

Treatment of Invasive Infection

Treatment of invasive C. acnes infection differs from routine acne therapy.

Management depends on:

• Infection site

• Presence of prosthetic material

• Antimicrobial susceptibility

• Severity

• Ability to remove or revise infected hardware

Because prosthetic infections involve biofilms, antimicrobial therapy alone may not always be sufficient.

Source Control

For device-associated disease, an important management principle is:

ANTIBIOTICS

• ●

DEVICE/SURGICAL SOURCE CONTROL

Depending on the infection, this may involve:

• Prosthesis revision

• Removal or replacement of an infected CSF shunt

• Debridement

• Drainage of an abscess

• Management of infected cardiac prosthetic material

Additional Antimicrobial Agents

The source lists:

• Trimethoprim–sulfamethoxazole

• Vancomycin

as additional treatment options.

Definitive treatment for serious invasive disease should be based on the:

Site of infection, susceptibility profile, and presence of prosthetic material.

Metronidazole Resistance

A particularly important microbiologic treatment clue is:

Cutibacterium acnes is resistant to metronidazole.

This is noteworthy because metronidazole is active against many anaerobic bacteria.

Therefore:

Anaerobic organism ≠ automatically susceptible to metronidazole

High-Yield Resistance Pattern

Anaerobic Gram-positive bacillus

• ●

Cutibacterium acnes

→ Metronidazole resistance

Prevention

Because many invasive infections are associated with surgical implantation of devices, prevention emphasizes:

Good antiseptic technique during surgery

This is particularly important because C. acnes normally resides within:

Sebaceous follicles of the skin

and can potentially contaminate the operative field.

Propionibacterium/Cutibacterium vs. Corynebacterium

Both may appear as:

Gram-positive bacilli

and both are associated with:

Skin flora

However:

Cutibacterium acnes

→ Anaerobic/aerotolerant

→ Slow growing

→ Acne

→ Strong association with prosthetic shoulder infection

Corynebacterium

→ Generally aerobic or facultative

→ Characteristic coryneform morphology

→ Includes both normal skin flora and important pathogens

Cutibacterium vs. Staphylococcus epidermidis

Both are important causes of:

Prosthetic-device infection

and both can form:

Biofilms

Cutibacterium acnes

→ Gram-positive bacillus

→ Slow-growing anaerobic/aerotolerant organism

→ Particularly associated with shoulder prostheses

Staphylococcus epidermidis

→ Gram-positive coccus

→ Coagulase-negative staphylococcus

→ Major cause of infections involving many types of indwelling devices

High-Yield Clinical Pattern

Shoulder arthroplasty

• ●

Chronic pain or prosthetic dysfunction

• ●

Minimal systemic inflammation

• ●

Slow-growing anaerobic Gram-positive bacillus

→ Think Cutibacterium acnes

Exam Essentials

Historical genus: Propionibacterium

Current important genus: Cutibacterium

Historical species: Propionibacterium acnes

Current name: Cutibacterium acnes

Type: Gram-positive bacillus

Growth: Slow-growing, anaerobic/aerotolerant

Normal habitat: Skin, particularly sebaceous follicles

Incubation: Unknown; invasive infections are usually endogenous

Classic superficial association: Acne vulgaris

Major invasive association: Prosthetic/device infection

Classic orthopedic association: Shoulder arthroplasty infection

Other device infections: CSF shunts and prosthetic cardiac material

Other diseases: Post-neurosurgical meningitis, brain abscess, bacteremia, endocarditis

Virulence feature: Biofilm formation

Diagnosis: Culture, often requiring anaerobic conditions and adequate incubation

Diagnostic challenge: Distinguishing contamination from true infection

Source acne treatment: Tetracycline or macrolide

Additional source agents: TMP-SMX and vancomycin

Important resistance: Metronidazole resistant

Device infection principle: Antimicrobial therapy + appropriate source control

Prevention: Careful surgical skin antisepsis and aseptic technique

Key clinical pearl: The organism historically called Propionibacterium acnes is now Cutibacterium acnes. Although it is common normal skin flora and may contaminate cultures, it is an important true pathogen in indolent prosthetic and device-associated infections—particularly shoulder arthroplasty infections. Its ability to form biofilms helps explain persistent hardware-associated disease, and a particularly useful antimicrobial clue is its resistance to metronidazole.

Infectious Disease and Microbiology – Prions

Overview

Prions are unusual infectious agents composed primarily of abnormally folded protein rather than conventional microorganisms. Unlike bacteria, viruses, fungi, and parasites, prions contain no known nucleic acid genome yet are capable of transmitting disease by inducing normally folded host proteins to adopt an abnormal conformation.

Prion diseases are collectively known as transmissible spongiform encephalopathies (TSEs). They are characterized by very long incubation periods, progressive neurodegeneration, spongiform changes in the brain, minimal inflammatory response, and ultimately death.

Classification

Genus: Not applicable

Species: Not applicable

Agent: Proteinaceous infectious particle (prion)

Major target organ: Central nervous system

Prions are fundamentally different from conventional infectious organisms because they lack:

DNA

and

RNA

Microbiologic Characteristics

Prions consist of an abnormal form of a host protein known as:

Prion protein (PrP)

The normal cellular form is commonly designated:

PrPᶜ

The abnormal disease-associated form has traditionally been designated:

PrPˢᶜ

The abnormal protein can promote conformational conversion of normal prion protein into additional abnormal molecules.

Basic Pathogenesis

Normal PrPᶜ

↓

Contact with abnormal prion protein

↓

Protein misfolding

↓

Conversion to abnormal prion conformation

↓

Accumulation and propagation of abnormal protein

↓

Neuronal dysfunction and degeneration

↓

Spongiform encephalopathy

High-Yield Microbiology Pattern

Infectious protein

• ●

No DNA or RNA

• ●

Progressive neurodegeneration

• ●

Spongiform brain changes

→ Think PRION DISEASE

Incubation Period

Prion diseases characteristically have:

Very long incubation periods

The interval between acquisition and clinical disease may extend for:

Years or even decades

depending on the specific prion disease and route of acquisition.

Once neurologic manifestations become clinically apparent, however, many prion diseases progress relentlessly.

Pathology

The characteristic neuropathologic process includes:

• Spongiform degeneration

• Neuronal loss

• Accumulation of abnormal prion protein

• Gliosis

• Minimal or absent conventional inflammatory response

The microscopic vacuolation gives affected brain tissue a:

Sponge-like appearance

hence the term:

Spongiform encephalopathy

Important Pathology Pattern

Rapid or progressive neurologic deterioration

• ●

Spongiform degeneration

• ●

Scant inflammatory response

→ Strongly suggests prion disease

Major Human Prion Diseases

Important human prion diseases include:

• Creutzfeldt–Jakob disease (CJD)

• Variant Creutzfeldt–Jakob disease (vCJD)

• Kuru

• Gerstmann–Sträussler–Scheinker syndrome (GSS)

• Fatal familial insomnia (FFI)

Human prion diseases can arise through:

Sporadic

Inherited

or

Acquired

mechanisms.

Sporadic Creutzfeldt–Jakob Disease

Sporadic CJD is the most common form of human prion disease.

It occurs without an identifiable external exposure or inherited pathogenic variant.

Typical manifestations include:

• Rapidly progressive dementia

• Myoclonus

• Ataxia

• Behavioral or cognitive changes

• Visual disturbances

• Pyramidal or extrapyramidal abnormalities

• Progression to severe neurologic disability

Classic CJD Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Ataxia

• ●

Characteristic MRI/EEG or CSF findings

→ Think Creutzfeldt–Jakob disease

Familial Prion Disease

Some prion diseases result from pathogenic variants involving:

PRNP

the gene encoding prion protein.

Inherited forms include:

• Familial CJD

• Gerstmann–Sträussler–Scheinker syndrome

• Fatal familial insomnia

Variant Creutzfeldt–Jakob Disease

Variant CJD (vCJD) became a major public health concern because of its association with exposure to the agent responsible for:

Bovine spongiform encephalopathy (BSE)

also known as:

“Mad cow disease.”

Variant CJD is distinct from the much more common sporadic form of CJD.

High-Yield Association

Bovine spongiform encephalopathy

→ Human exposure

→ Variant Creutzfeldt–Jakob disease

Kuru

Kuru is an acquired human prion disease historically identified among the Fore people of Papua New Guinea.

Transmission was associated with ritual mortuary practices involving consumption of tissues from deceased individuals.

The disease became a landmark example demonstrating the:

Transmissibility of prion disease

Gerstmann–Sträussler–Scheinker Syndrome

Gerstmann–Sträussler–Scheinker syndrome (GSS) is a rare:

Inherited prion disease

It typically produces progressive neurologic deterioration, often with prominent:

Cerebellar ataxia

and later cognitive decline.

Fatal Familial Insomnia

Fatal familial insomnia (FFI) is an inherited prion disorder characterized by prominent involvement of the:

Thalamus

The syndrome includes progressively severe:

Insomnia

along with autonomic and neurologic abnormalities.

The source’s description of:

“Insomnia secondary to thalamic destruction”

corresponds particularly well to this disorder.

High-Yield Pattern

Progressive severe insomnia

• ●

Autonomic dysfunction

• ●

Thalamic degeneration

• ●

Family history

→ Think Fatal familial insomnia

Clinical Course

Despite differences between individual prion diseases, the overall pattern is:

Long incubation

↓

Clinical neurologic manifestations begin

↓

Progressive neuronal degeneration

↓

Worsening cognitive and motor dysfunction

↓

Severe neurologic disability

↓

Death

Prion diseases are currently considered progressive and fatal.

Inflammatory Response

Unlike many infectious diseases, prion disease generally produces:

Little conventional inflammatory response

This is an important pathologic characteristic.

Thus:

Profound neurodegeneration

can occur despite:

Scant inflammation

Diagnosis

Modern diagnosis of suspected prion disease uses a combination of:

• Clinical presentation

• Brain MRI

• Cerebrospinal fluid testing

• EEG in appropriate cases

• Definitive neuropathologic evaluation when available

CSF RT-QuIC

An especially important modern diagnostic test is:

RT-QuIC

(real-time quaking-induced conversion)

This assay detects the ability of abnormal prion protein in a patient’s specimen to induce conformational changes in substrate prion protein.

CSF RT-QuIC has become an important test for supporting the diagnosis of:

Sporadic CJD

High-Yield Diagnostic Update

Suspected CJD

→ MRI brain

• ●

CSF RT-QuIC

with other clinical and laboratory findings used to establish diagnostic probability.

CSF 14-3-3 Protein

The source lists:

CSF 14-3-3 protein

as a diagnostic finding.

Elevated CSF 14-3-3 can support the diagnosis in an appropriate clinical setting, but it reflects:

Rapid neuronal injury

and is therefore not specific for prion disease.

It can be elevated in other neurologic disorders associated with extensive neuronal damage.

MRI

Brain MRI is extremely useful in suspected CJD.

Characteristic abnormalities may involve:

Cerebral cortex

and

Basal ganglia

with diffusion abnormalities being particularly important.

A classic MRI description in sporadic CJD is:

Cortical ribboning

High-Yield MRI Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Cortical ribboning on diffusion-weighted MRI

→ Strongly consider CJD

EEG

EEG may demonstrate characteristic abnormalities in some patients with sporadic CJD, classically:

Periodic sharp-wave complexes

However, EEG findings are not present in every patient and are interpreted together with MRI, CSF, and clinical findings.

Direct Visualization

Older descriptions may emphasize direct visualization of prion-associated material by:

Electron microscopy

However, electron microscopy is not the routine modern diagnostic approach for suspected CJD.

Modern evaluation relies much more heavily on:

MRI + CSF RT-QuIC + clinical findings

with neuropathology providing definitive confirmation in appropriate circumstances.

Neuropathology

Brain tissue examination may demonstrate:

• Spongiform change

• Neuronal loss

• Gliosis

• Abnormal prion protein deposition

Detection of disease-associated prion protein can provide strong diagnostic evidence.

Treatment

There is currently:

No established curative treatment

for human prion diseases.

Management is primarily:

Supportive and palliative

Supportive Management

Care may include management of:

• Myoclonus

• Pain or discomfort

• Psychiatric or behavioral manifestations

• Sleep disturbances

• Nutritional needs

• Mobility impairment

• Progressive neurologic disability

Because disease is progressive, supportive care and planning become increasingly important.

Infection-Control Importance

Prions are unusually resistant to many conventional:

Disinfection and sterilization procedures

This is especially relevant for instruments that contact:

High-infectivity nervous system tissues

Standard sterilization procedures may not be sufficient under circumstances involving suspected prion contamination, so specialized infection-control protocols are required.

Prions vs. Viruses

Prions

→ Protein only

→ No known DNA or RNA genome

→ Propagate through protein conformational conversion

→ Cause spongiform neurodegeneration

→ Extremely long incubation possible

Viruses

→ Contain DNA or RNA

→ Encode viral genetic information

→ Replicate using host-cell machinery

→ Produce a much broader spectrum of disease

CJD vs. Alzheimer Disease

Creutzfeldt–Jakob disease

→ Usually rapidly progressive cognitive decline

→ Myoclonus common

→ Ataxia may occur

→ Characteristic MRI abnormalities

→ CSF RT-QuIC may be positive

→ Progression often occurs over months

Alzheimer disease

→ Usually slowly progressive over years

→ Memory impairment prominent early

→ Myoclonus is not a typical early feature

→ Different pathologic protein abnormalities

Thus, the speed of progression is an important clinical clue.

CJD vs. Autoimmune Encephalitis

Both can cause:

Rapidly progressive cognitive and neurologic deterioration

However, autoimmune encephalitis is particularly important in the differential because some forms are:

Potentially treatable

Therefore, suspected CJD requires careful evaluation for alternative causes of rapidly progressive dementia.

Major Prion Disease Associations

Sporadic CJD

→ Most common human prion disease

Variant CJD

→ Associated with BSE

Kuru

→ Historical ritual exposure in Papua New Guinea

Gerstmann–Sträussler–Scheinker syndrome

→ Inherited, often prominent ataxia

Fatal familial insomnia

→ Inherited, prominent insomnia and thalamic degeneration

High-Yield Clinical Pattern

Rapidly progressive dementia

• ●

Myoclonus

• ●

Ataxia

• ●

Cortical ribboning on MRI

• ●

Positive CSF RT-QuIC

→ Think CREUTZFELDT–JAKOB DISEASE

Exam Essentials

Agent: Prion

Composition: Abnormally folded proteinaceous infectious material

Nucleic acid: Absent

Mechanism: Induces misfolding of normal host prion protein

Disease group: Transmissible spongiform encephalopathies

Incubation: Usually very long—potentially years to decades

Target: Central nervous system

Pathology: Spongiform degeneration + neuronal loss + gliosis

Inflammation: Characteristically minimal

Course: Progressive and ultimately fatal

Most common human prion disease: Sporadic CJD

CJD clue: Rapidly progressive dementia + myoclonus

Important MRI clue: Cortical ribboning

Important modern CSF test: RT-QuIC

Older/supportive CSF marker: 14-3-3 protein

Classic EEG: Periodic sharp-wave complexes

BSE association: Variant CJD

Papua New Guinea association: Kuru

Inherited ataxic syndrome: Gerstmann–Sträussler–Scheinker syndrome

Thalamic degeneration + insomnia: Fatal familial insomnia

Treatment: No established curative therapy; supportive care

Infection control: Prions have unusual resistance to conventional decontamination procedures

Key clinical pearl: Prions are infectious misfolded proteins that contain no nucleic acid yet propagate by inducing abnormal folding of normal host prion protein. The classic clinical syndrome is a rapidly progressive, fatal neurodegenerative disorder with spongiform brain changes and minimal inflammation; for sporadic CJD, high-yield modern diagnostic clues include cortical ribboning on MRI and a positive CSF RT-QuIC assay.

Infectious Disease and Microbiology – Plesiomonas shigelloides

Overview

Plesiomonas shigelloides is a Gram-negative bacillus found primarily in freshwater environments and soil. Human infection is most commonly associated with gastroenteritis, particularly after exposure to contaminated water or food, and it is recognized as a potential cause of traveler’s diarrhea.

Most intestinal infections are mild and self-limited, but some patients develop an inflammatory or dysentery-like illness with bloody diarrhea, fever, malaise, and fecal leukocytes. Rarely, P. shigelloides causes invasive extraintestinal disease, including bacteremia, soft tissue infection, bone and joint infection, and meningitis.

Classification

Genus: Plesiomonas

Species: Plesiomonas shigelloides

Organism type: Gram-negative bacillus

Microbiologic Characteristics

P. shigelloides is a:

• Gram-negative bacillus

• Facultatively anaerobic organism

• Oxidase-positive bacterium

• Motile organism

• Freshwater-associated enteric pathogen

It is commonly grouped clinically with other water-associated Gram-negative organisms that can produce gastrointestinal disease.

High-Yield Microbiology Pattern

Gram-negative bacillus

• ●

Freshwater exposure

• ●

Acute diarrhea

→ Consider Plesiomonas shigelloides

Incubation Period

The precise incubation period is:

Not clearly established

Evidence from cases of traveler’s diarrhea suggests that symptoms generally develop after a:

Short incubation period of several days

following exposure.

Epidemiology

P. shigelloides is widely distributed in:

Freshwater

and

Soil

It can also be associated with aquatic environments and animals.

Human infection is generally acquired through environmental or food-related exposure rather than sustained person-to-person transmission.

Transmission

Gastrointestinal infection is associated with ingestion of:

• Contaminated water

• Contaminated food

• Raw or inadequately cooked aquatic foods in some settings

Exposure during international travel may lead to:

Traveler’s diarrhea

High-Yield Exposure Pattern

Recent travel

• ●

Freshwater or contaminated food/water exposure

• ●

Acute diarrhea

→ Consider Plesiomonas shigelloides

Gastroenteritis

The most common clinical manifestation is:

Acute gastroenteritis

Disease severity varies considerably.

Some patients develop:

Mild, watery, self-limited diarrhea

while others develop a more inflammatory illness.

Mild Diarrheal Disease

Uncomplicated infection may present with:

• Watery diarrhea

• Abdominal discomfort or cramping

• Nausea

• Malaise

Most mild infections resolve without specific antimicrobial therapy.

Severe or Dysentery-Like Disease

More severe P. shigelloides gastroenteritis may produce:

Bloody diarrhea

associated with:

• Fever

• Malaise

• Abdominal symptoms

• White blood cells in the stool

The presence of blood and fecal leukocytes suggests an:

Inflammatory diarrheal syndrome

High-Yield Clinical Pattern

Traveler

• ●

Acute diarrhea

• ●

Blood and fecal leukocytes

• ●

Freshwater/environmental exposure

→ Consider Plesiomonas shigelloides

Traveler’s Diarrhea

P. shigelloides is one recognized cause of:

Traveler’s diarrhea

Although other organisms are considerably more common, Plesiomonas should be considered when the epidemiologic history includes:

Travel + contaminated food/water + acute gastroenteritis

Extraintestinal Infection

Although gastrointestinal disease is the major manifestation, P. shigelloides can occasionally produce:

Invasive extraintestinal infections

These are much less common than diarrhea.

Bacteremia

Rarely, the organism may enter the bloodstream and cause:

Bacteremia

Systemic infection is more concerning in vulnerable patients and may require antimicrobial treatment.

Skin and Subcutaneous Tissue Infection

The source also reports:

Skin and subcutaneous tissue infections

These may be particularly relevant when damaged tissue is exposed to contaminated environmental water.

Septic Arthritis and Osteomyelitis

Rare musculoskeletal manifestations include:

Septic arthritis

and

Osteomyelitis

These invasive infections require:

Systemic antimicrobial therapy

and may also require drainage or other source-control procedures.

Other Rare Infections

Additional reported manifestations include:

• Cholecystitis

• Endophthalmitis

• Meningitis

These infections are uncommon but demonstrate that P. shigelloides can occasionally behave as an invasive pathogen outside the gastrointestinal tract.

Neonatal Meningitis

One of the most important severe manifestations is:

Meningitis in neonates

Although rare, neonatal P. shigelloides meningitis can be a serious invasive infection.

High-Yield Severe Disease Pattern

Neonate

• ●

Gram-negative bacillary meningitis

• ●

Possible environmental/water-associated organism

→ Consider Plesiomonas shigelloides among rare causes

Diagnosis

The principal diagnostic method is:

Culture

For gastrointestinal disease, the organism can be recovered from:

Stool culture

For invasive disease, specimens depend on the affected site and may include:

• Blood

• Cerebrospinal fluid

• Joint fluid

• Wound or tissue specimens

• Other normally sterile fluids

Laboratory Identification

Laboratory identification is important because P. shigelloides may resemble other enteric Gram-negative organisms.

A useful characteristic is:

Oxidase positivity

This helps distinguish it from many members of the:

Enterobacterales

which are typically oxidase negative.

High-Yield Laboratory Pattern

Enteric-appearing Gram-negative bacillus

• ●

Oxidase positive

• ●

Freshwater association

• ●

Diarrheal illness

→ Think Plesiomonas shigelloides

Treatment

For uncomplicated gastroenteritis:

Antibiotics are usually unnecessary

because many infections are:

Mild and self-limited

The cornerstone of treatment is:

Fluid and electrolyte replacement

Rehydration

Management should focus on correcting losses caused by diarrhea.

This includes:

Oral rehydration

when tolerated.

More severe dehydration may require:

Intravenous fluid and electrolyte replacement

Antimicrobial Therapy

The source lists a:

Fluoroquinolone

as an antimicrobial treatment option.

Antibiotic therapy may be considered particularly for:

• Severe disease

• Persistent diarrhea

• Dysentery-like illness

• Significant systemic symptoms

• Invasive extraintestinal infection

Selection should be guided by the clinical syndrome and susceptibility information when available.

Additional Treatment Options

The source lists:

• Trimethoprim–sulfamethoxazole

• Chloramphenicol

• Carbapenems

• Aminoglycosides

For serious invasive infection, therapy should be selected according to:

Culture and antimicrobial susceptibility results

whenever possible.

Source Control

For focal invasive infections such as:

Septic arthritis

or

Deep soft tissue infection

antimicrobial therapy may need to be combined with:

Drainage or surgical source control

when clinically indicated.

Plesiomonas vs. Shigella

Despite the species name:

shigelloides

Plesiomonas shigelloides is not a Shigella species.

Plesiomonas shigelloides

→ Freshwater-associated

→ Oxidase positive

→ Motile

→ Can cause watery or bloody diarrhea

Shigella

→ Primarily human intestinal pathogen

→ Oxidase negative

→ Nonmotile

→ Characteristically causes inflammatory dysentery

Plesiomonas vs. Aeromonas

Both organisms may be associated with:

Freshwater

and can cause:

Gastrointestinal disease

Plesiomonas shigelloides

→ Diarrhea, including traveler’s diarrhea

→ Rare invasive infections

→ Neonatal meningitis is a notable severe manifestation

Aeromonas species

→ Freshwater exposure

→ Gastroenteritis

→ Particularly important in wound and soft tissue infections after water exposure

Plesiomonas vs. Vibrio

Plesiomonas

→ Classically associated with freshwater

→ Traveler’s diarrhea possible

Vibrio

→ Many clinically important species are strongly associated with marine or brackish water

→ Raw seafood exposure is an important epidemiologic clue for several species

Thus, the type of water exposure can help narrow the differential diagnosis.

Prevention

Prevention centers on reducing exposure to contaminated food and water.

Important measures include:

• Drinking safe water

• Appropriate food hygiene

• Properly cooking aquatic foods

• Careful food and water practices during travel

• Hand hygiene

High-Yield Clinical Pattern

Freshwater exposure

• ●

Short incubation of a few days

• ●

Traveler’s diarrhea

• ●

Watery or bloody diarrhea

• ●

Oxidase-positive Gram-negative bacillus

→ Think Plesiomonas shigelloides

Exam Essentials

Genus: Plesiomonas

Species: P. shigelloides

Type: Gram-negative bacillus

Metabolism: Facultatively anaerobic

Oxidase: Positive

Motility: Motile

Reservoir: Freshwater and soil

Incubation: Uncertain; probably a few days in many cases

Major disease: Gastroenteritis

Diarrhea: May range from mild watery disease to bloody inflammatory diarrhea

Stool finding in severe disease: Fecal leukocytes may occur

Travel association: Cause of traveler’s diarrhea

Rare infections: Bacteremia, skin/soft tissue infection, septic arthritis, osteomyelitis, cholecystitis, endophthalmitis, and meningitis

Important severe association: Neonatal meningitis

Diagnosis: Culture

Mild infection: Usually supportive treatment only

Essential therapy: Fluids and electrolytes

Source antimicrobial: Fluoroquinolone

Additional source agents: TMP-SMX, chloramphenicol, carbapenems, and aminoglycosides

Important laboratory clue: Oxidase positive, unlike typical Enterobacterales

Major environmental clue: Freshwater exposure

Key clinical pearl: Plesiomonas shigelloides is a freshwater-associated, oxidase-positive Gram-negative bacillus that primarily causes gastroenteritis, including traveler’s diarrhea. Most cases require only fluid and electrolyte replacement, but bloody inflammatory diarrhea can occur, and rare invasive infections—including particularly serious neonatal meningitis—should be recognized.

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Infectious Disease and Microbiology – Peptostreptococcus Species

Overview

Peptostreptococcus species are anaerobic Gram-positive cocci that form part of the normal human flora of the mouth, gastrointestinal tract, and female genital tract. They usually cause endogenous infections when normal mucosal barriers are disrupted and organisms enter normally sterile tissues.

They are especially important in polymicrobial infections, including intra-abdominal, pelvic, lung, and brain abscesses, but they may also cause bacteremia, surgical wound infection, skin and soft tissue infection, septic arthritis, and rarely endocarditis.

Classification

Genus: Peptostreptococcus

Species listed in the source include:

• P. anaerobius

• P. asaccharolyticus

• P. magnus

• P. prevotii

• Other species

Organism type: Anaerobic Gram-positive coccus

Taxonomy Note

The taxonomy of anaerobic Gram-positive cocci has changed substantially over time.

Some organisms historically classified as Peptostreptococcus have subsequently been moved into other genera. For example, organisms formerly known as:

Peptostreptococcus magnus

are now generally classified as:

Finegoldia magna

Therefore, older microbiology references may use names that differ from current laboratory terminology.

Microbiologic Characteristics

Peptostreptococcus species are:

• Gram-positive cocci

• Anaerobic

• Part of normal mucosal flora

• Usually involved in endogenous infections

• Frequently recovered together with other aerobic and anaerobic organisms

They are particularly associated with infections occurring in tissues with:

Low oxygen tension

and

Abscess formation

High-Yield Microbiology Pattern

Anaerobic Gram-positive coccus

• ●

Normal oral, bowel, or vaginal flora

• ●

Polymicrobial abscess

→ Think Peptostreptococcus

Incubation Period

A specific incubation period is generally not defined because infection usually results from:

Endogenous spread

rather than transmission from another person.

The organism is already present as part of the patient’s normal flora and becomes pathogenic after disruption of normal barriers.

Epidemiology

Peptostreptococcus species occur:

Worldwide

They normally colonize:

• Mouth and oropharynx

• Gastrointestinal tract

• Female genital tract

Because they are normal flora, the source of infection is usually:

The patient’s own microbiota

Pathogenesis

A typical sequence is:

Normal mucosal colonization

↓

Mucosal injury, surgery, aspiration, perforation, or tissue damage

↓

Organisms enter normally sterile tissue

↓

Low-oxygen environment develops

↓

Growth of anaerobic bacteria

↓

Polymicrobial infection and abscess formation

Polymicrobial Infection

Peptostreptococcus species are frequently isolated together with:

Aerobic bacteria

and

Other anaerobic bacteria

This is especially common in infections arising from:

• Oral flora

• Bowel flora

• Genital tract flora

Therefore, treatment often needs to cover:

Multiple bacterial groups

rather than Peptostreptococcus alone.

Intra-Abdominal Abscess

A major clinical setting is:

Intra-abdominal infection

Possible situations include:

• Bowel perforation

• Appendiceal disease

• Postoperative infection

• Intra-abdominal abscess formation

These infections are typically:

Polymicrobial

with combinations of enteric Gram-negative organisms and anaerobes.

High-Yield Pattern

GI source

• ●

Intra-abdominal abscess

• ●

Mixed aerobic/anaerobic culture

→ Consider Peptostreptococcus among the anaerobic organisms

Pelvic Infection

Peptostreptococcus may participate in:

Pelvic abscesses and polymicrobial pelvic infections

because it is part of the normal:

Female genital tract flora

Potential clinical settings include postoperative infection and other conditions that disrupt pelvic or genital tract barriers.

Lung Abscess

Because Peptostreptococcus can colonize the mouth and upper respiratory tract, aspiration may introduce these organisms into the lower respiratory tract.

This can contribute to:

Aspiration-associated pneumonia

and

Lung abscess

Anaerobic Pulmonary Pattern

Aspiration risk

• ●

Poor dentition/oral flora exposure

• ●

Necrotizing pneumonia or lung abscess

→ Think oral anaerobes, including Peptostreptococcus

Brain Abscess

Peptostreptococcus species can also occur in:

Brain abscesses

particularly when infection spreads from:

• Dental disease

• Sinus infection

• Otogenic infection

• Other contiguous sources

Brain abscesses can be polymicrobial, so appropriate anaerobic cultures are important.

Bacteremia

The organism may cause:

Bacteremia

especially when there is:

• A deep abscess

• Tissue necrosis

• Gastrointestinal or pelvic infection

• Significant mucosal disruption

Anaerobic bacteremia should prompt evaluation for an underlying deep infectious source.

Endocarditis

The source notes that Peptostreptococcus is a:

Rare cause of endocarditis

When endocarditis occurs, management may require:

• Prolonged antimicrobial therapy

• Blood cultures

• Echocardiographic evaluation

• Assessment for valvular complications

Skin and Soft Tissue Infection

Peptostreptococcus may contribute to:

Skin and soft tissue infections

especially when:

• Tissue oxygenation is poor

• Necrosis is present

• Infection is polymicrobial

• There is contamination from mucosal flora

Surgical Wound Infection

Postoperative wounds may become infected with mixed organisms, including:

Anaerobic Gram-positive cocci

particularly after procedures involving:

• Gastrointestinal tract

• Pelvic organs

• Contaminated tissue planes

Septic Arthritis

The source also lists:

Septic arthritis

as a possible manifestation.

This is uncommon but clinically significant because joint infection may require:

Joint drainage

plus

Appropriate antimicrobial therapy

Diagnosis

The primary diagnostic method is:

Culture under anaerobic conditions

Appropriate specimens include:

• Abscess aspirates

• Deep tissue samples

• Blood

• Joint fluid

• Normally sterile body fluids

Specimen Collection

Because these organisms are anaerobic:

Specimen quality and transport are critical

Whenever possible, obtain:

Deep aspirated material or tissue

rather than superficial swabs.

Specimens should be transported under:

Anaerobic conditions

to maximize recovery.

Interpretation of Culture

Because Peptostreptococcus species can be part of normal flora, interpretation depends on:

• Site of isolation

• Clinical syndrome

• Presence of an abscess

• Whether the specimen is from a sterile site

• Other organisms recovered

• Evidence of tissue invasion

Isolation from a properly obtained deep specimen in a compatible infection supports true pathogenicity.

Treatment

The source lists:

Penicillin G

as the principal treatment.

Historically, many anaerobic Gram-positive cocci have been susceptible to penicillin.

However, definitive treatment should consider:

Species identification

and

Antimicrobial susceptibility

when available.

Additional Treatment Options

The source lists:

• Clindamycin

• Cephamycins

• Carbapenems, including imipenem and meropenem

• Vancomycin

Because these infections are frequently polymicrobial, broader antimicrobial coverage may be required.

Broad-Spectrum Therapy

For serious polymicrobial infections, treatment may need activity against:

Anaerobes

• ●

Enteric Gram-negative bacilli

• ●

Gram-positive organisms

This is particularly relevant in:

• Intra-abdominal infection

• Pelvic infection

• Aspiration-associated lung abscess

• Necrotizing soft tissue infection

Source Control

A central treatment principle is:

ANTIBIOTICS

• ●

SOURCE CONTROL

Examples include:

• Drainage of abscesses

• Debridement of infected or necrotic tissue

• Joint drainage in septic arthritis

• Correction of perforation or other anatomic defects

• Management of infected surgical sites

High-Yield Treatment Principle

Anaerobic polymicrobial abscess

→ Appropriate antimicrobial therapy

AND

→ Drainage/source control when indicated

Peptostreptococcus vs. Peptococcus

Both are historically classified as:

Anaerobic Gram-positive cocci

and both may be part of:

Normal human flora

with disease characterized by:

• Endogenous infection

• Abscess formation

• Polymicrobial disease

A major distinction is taxonomic rather than a simple bedside clinical difference, and older sources may use these genera differently from current microbiology laboratories.

Peptostreptococcus vs. Staphylococcus aureus

Peptostreptococcus

→ Anaerobic

→ Normal mucosal flora

→ Often polymicrobial

→ Frequently associated with deep abscesses

Staphylococcus aureus

→ Facultative anaerobic Gram-positive coccus

→ Often a primary pathogen

→ Common cause of skin abscesses, bacteremia, endocarditis, and osteoarticular infection

Prevention

Because these infections are usually:

Endogenous

there is no specific vaccine or person-to-person avoidance strategy.

Prevention focuses on:

• Appropriate surgical technique

• Good oral hygiene

• Prompt management of perforation or tissue necrosis

• Proper wound care

• Prevention of aspiration in high-risk patients

• Early drainage of developing abscesses

High-Yield Clinical Pattern

Anaerobic Gram-positive cocci

• ●

Normal mouth, bowel, or vaginal flora

• ●

Polymicrobial infection

• ●

Abscess

→ Think Peptostreptococcus

High-Yield Site Pattern

Oral flora

→ Lung abscess / brain abscess

Bowel flora

→ Intra-abdominal abscess

Vaginal flora

→ Pelvic infection

This helps connect normal colonization sites with the likely infectious syndrome.

Exam Essentials

Genus: Peptostreptococcus

Source-listed species: P. anaerobius, P. asaccharolyticus, P. magnus, and P. prevotii

Type: Anaerobic Gram-positive coccus

Normal habitat: Mouth, gastrointestinal tract, and vagina

Distribution: Worldwide

Transmission: Usually endogenous

Major microbiologic setting: Polymicrobial infection

Major infections: Intra-abdominal, pelvic, lung, and brain abscesses

Other infections: Bacteremia, skin/soft tissue infection, surgical wound infection, septic arthritis, and rare endocarditis

Diagnosis: Anaerobic culture

Source treatment: Penicillin G

Additional source treatments: Clindamycin, cephamycin, imipenem, meropenem, and vancomycin

Major management principle: Treat the entire polymicrobial infection

Source control: Often essential for abscesses and septic arthritis

Taxonomy note: Some older Peptostreptococcus species have been reassigned to other genera

Key clinical pearl: Peptostreptococcus species are anaerobic Gram-positive cocci that normally inhabit the mouth, bowel, and female genital tract. They become pathogenic when normal barriers are disrupted and are most characteristic of endogenous polymicrobial abscesses—especially intra-abdominal, pelvic, lung, and brain abscesses—where successful management often requires both appropriate anaerobic antimicrobial coverage and source control.