Infectious Disease and Microbiology – Serratia Species

Overview

Serratia species are Gram-negative bacilli belonging to the order Enterobacterales. Although several species can occasionally cause human disease, Serratia marcescens is by far the most clinically important.

Serratia is particularly associated with healthcare-associated infections, especially in hospitalized patients exposed to invasive procedures, urinary or vascular catheters, mechanical ventilation, or prolonged antimicrobial therapy. Important syndromes include bacteremia, urinary tract infection, pneumonia, intra-abdominal infection, wound infection, and device-associated infection.

Classification

Genus: Serratia

Important species include:

• Serratia ficaria

• Serratia fonticola

• Serratia grimesii

• Serratia liquefaciens

• Serratia marcescens — most clinically relevant species

• Serratia odorifera

• Serratia plymuthica

• Serratia rubidaea

• Other Serratia species

Microbiologic Characteristics

Serratia species are:

• Gram-negative bacilli

• Members of Enterobacterales

• Facultatively anaerobic, although the source describes them as aerobic

• Generally oxidase-negative

• Usually motile

• Capable of surviving in moist environmental and healthcare settings

These properties contribute to their ability to colonize:

Hospital environments + medical equipment + fluids

Serratia marcescens

Serratia marcescens is the major human pathogen within the genus.

It is an important:

Opportunistic nosocomial Gram-negative bacillus

and can produce outbreaks in:

• Intensive care units

• Neonatal units

• Surgical wards

• Other healthcare environments

Red Pigmentation

A classic microbiologic feature of some S. marcescens strains is production of the red pigment:

PRODIGIOSIN

This can produce:

Pink to red colonies

under appropriate growth conditions.

However, an important practical point is that:

Not all clinical isolates produce visible red pigment.

Therefore, absence of red pigmentation does not exclude S. marcescens.

High-Yield Microbiology Pattern

Gram-negative bacillus

• ●

Healthcare-associated infection

• ●

Possible red pigment (prodigiosin)

→ Think Serratia marcescens

Epidemiology

Serratia species have a:

Worldwide distribution

The organism is found in environmental reservoirs, particularly:

Water and moist environments

Its ability to survive under relatively nutrient-poor conditions contributes to its importance in healthcare-associated transmission.

Nosocomial Infection

The source emphasizes that Serratia is commonly associated with:

NOSOCOMIAL INFECTION

Major risk factors include:

• Hospitalization

• Intensive care exposure

• Invasive procedures

• Central venous catheters

• Urinary catheters

• Mechanical ventilation

• Surgery

• Broad-spectrum antibiotic exposure

• Significant underlying illness

High-Yield Nosocomial Pattern

Hospitalized patient

• ●

Invasive device/procedure

• ●

Gram-negative bacteremia, UTI, or pneumonia

→ Consider Serratia marcescens

Intravenous Drug Use

Another important risk factor described in the source is:

Intravenous drug use

This is particularly relevant because bloodstream inoculation can produce:

Bacteremia

and potentially:

Infective endocarditis

Healthcare Reservoirs

Serratia can persist in moist healthcare environments and has historically been associated with contamination of:

• Solutions

• Medical equipment

• Respiratory equipment

• Sinks and water-associated environments

This ability contributes to:

Healthcare-associated outbreaks

Clinical Infections

Serratia can cause infection at multiple anatomical sites.

Major syndromes include:

Bacteremia

Urinary tract infection

Respiratory tract infection

Intra-abdominal infection

Other invasive infections may also occur.

Bacteremia

Serratia can cause:

Bloodstream infection

particularly in hospitalized patients with:

Central venous catheters or other invasive devices

Potential sources include:

• Urinary tract

• Respiratory tract

• Intra-abdominal infection

• Vascular catheter

• Wound or surgical infection

High-Yield Bacteremia Pattern

Hospitalized patient

• ●

Central venous catheter

• ●

Gram-negative bacteremia

• ●

S. marcescens cultured

→ Consider catheter-associated Serratia bloodstream infection

Urinary Tract Infection

Serratia is an important cause of:

Healthcare-associated UTI

particularly in patients with:

Indwelling urinary catheters

Clinical manifestations range from:

Catheter-associated bacteriuria

to:

Cystitis → pyelonephritis → urosepsis

Respiratory Tract Infection

Serratia may cause:

Hospital-acquired pneumonia

particularly in:

• Mechanically ventilated patients

• Critically ill patients

• Patients with prolonged hospitalization

• Patients with significant underlying disease

Ventilator-associated infection is an important healthcare-associated presentation.

Intra-Abdominal Infection

Serratia can also participate in:

Intra-abdominal infections

particularly following:

• Surgery

• Gastrointestinal disruption

• Invasive procedures

• Severe underlying illness

Such infections may be:

Polymicrobial

and require both antimicrobial therapy and appropriate source control.

Wound and Surgical-Site Infection

Serratia may infect:

Surgical wounds

or other disrupted tissue.

The organism should be considered particularly when infection develops in a healthcare setting or after exposure to contaminated medical material.

Endocarditis

Although less common than bacteremia or UTI, Serratia can cause:

Infective endocarditis

A classic epidemiologic association is:

Injection drug use

although healthcare-associated and device-related cases may also occur.

High-Yield Endocarditis Pattern

Injection drug use

• ●

Persistent Gram-negative bacteremia

• ●

Endocarditis

→ Consider Serratia marcescens

Other Invasive Infections

Less commonly, Serratia species can cause:

• Osteomyelitis

• Septic arthritis

• Meningitis

• Endophthalmitis

• Skin and soft-tissue infection

These manifestations generally occur in patients with relevant:

Healthcare exposure, trauma, devices, or impaired host defenses.

Diagnosis

The primary diagnostic method is:

CULTURE

Appropriate specimens depend on the clinical syndrome and may include:

• Blood

• Urine

• Respiratory specimens

• Wound material

• Intra-abdominal fluid

• CSF

• Other normally sterile specimens

Antimicrobial Susceptibility Testing

A particularly important component of diagnosis is:

ANTIMICROBIAL SUSCEPTIBILITY TESTING

Treatment should be selected according to:

Organism identification + infection site + severity + susceptibility results

because Serratia can possess important antimicrobial resistance mechanisms.

Antimicrobial Resistance

Serratia species possess clinically important resistance mechanisms.

S. marcescens has intrinsic resistance to several antimicrobial agents and may acquire additional resistance mechanisms.

Of particular concern are:

• β-lactamase-mediated resistance

• ESBL production in some isolates

• Carbapenemase production in some strains

• Multidrug-resistant healthcare-associated isolates

Therefore:

Susceptibility testing is essential.

AmpC β-Lactamase

S. marcescens possesses a chromosomal:

AmpC β-lactamase

This contributes to resistance to several β-lactam antibiotics.

The clinical importance of AmpC behavior varies among Enterobacterales, so definitive treatment should be guided by:

Current susceptibility results and the severity/site of infection.

Important Intrinsic Resistance

A classic high-yield feature is that Serratia is intrinsically resistant to agents such as:

Ampicillin

and

Early-generation cephalosporins

Therefore, these should not be assumed to provide reliable treatment.

High-Yield Resistance Pattern

Serratia marcescens

→ Healthcare-associated pathogen

→ Chromosomal AmpC β-lactamase

→ Possible additional acquired resistance

→ Always review susceptibility results

Treatment

The source appropriately emphasizes that treatment should be:

BASED ON SUSCEPTIBILITY DATA

There is no single antibiotic that should automatically be used for every Serratia infection.

Choice depends on:

Susceptibility + infection site + disease severity + resistance mechanism + patient factors

Fluoroquinolones

The source lists:

Ciprofloxacin

and other fluoroquinolones as potential treatments when the isolate is susceptible.

These agents may be useful for selected infections, particularly when:

Susceptibility is confirmed.

Cephalosporins

The source lists:

Third-generation cephalosporins

as potential treatment options.

Because resistance mechanisms may complicate β-lactam therapy, the specific agent should be chosen according to:

Susceptibility testing and clinical context.

Carbapenems

Potential agents include:

Meropenem

and

Imipenem

Carbapenems may be particularly relevant for serious infections caused by organisms with certain resistant β-lactamase phenotypes, provided the isolate remains:

Carbapenem susceptible

Other Potential Agents

Depending on susceptibility, the source also lists:

• Aztreonam

• Ureidopenicillins

• Aminoglycosides

• Ofloxacin

• Trimethoprim–sulfamethoxazole

The choice should not be made solely on the basis of species identification.

Source Control

Treatment of invasive Serratia infection frequently requires attention to:

SOURCE CONTROL

Examples include:

Central-line infection

→ Remove or replace implicated catheter when appropriate

Catheter-associated UTI

→ Remove or replace unnecessary urinary catheter

Intra-abdominal abscess

→ Drain infected collection

Obstruction

→ Correct anatomical obstruction

Infected prosthetic material

→ Evaluate need for device removal

High-Yield Management Pattern

Serratia infection

→ Obtain cultures

• ●

Perform susceptibility testing

• ●

Choose active antimicrobial therapy

• ●

Control/remove the infectious source

Serratia vs. Pseudomonas aeruginosa

Both are important:

Healthcare-associated Gram-negative pathogens

However:

Serratia marcescens

→ Enterobacterales

→ Oxidase negative

→ May produce red prodigiosin pigment

Pseudomonas aeruginosa

→ Nonfermenting Gram-negative bacillus

→ Oxidase positive

→ Characteristic blue-green pigments may occur

Serratia vs. Enterobacter

Both can cause:

Nosocomial UTI, pneumonia, and bacteremia

and both may have:

Chromosomal AmpC β-lactamases

However, the classic distinguishing clue for Serratia marcescens is:

Red prodigiosin pigmentation

although this feature is not present in every clinical isolate.

Serratia vs. Klebsiella

Serratia

→ Usually motile

→ Possible red pigmentation

→ Healthcare-associated opportunistic pathogen

Klebsiella

→ Nonmotile

→ Prominent polysaccharide capsule

→ Classically produces very mucoid colonies

Both may acquire:

ESBLs and carbapenemases

and cause multidrug-resistant healthcare-associated infections.

Prevention

Prevention focuses primarily on:

Healthcare infection-control practices

including:

• Hand hygiene

• Appropriate central-line care

• Appropriate urinary catheter care

• Proper respiratory equipment management

• Environmental cleaning

• Avoidance of unnecessary invasive devices

• Appropriate antimicrobial stewardship

Healthcare-associated clusters should raise concern for:

A common environmental or equipment-related source.

High-Yield Clinical Pattern

Hospitalized patient

• ●

Invasive catheter or procedure

• ●

Bacteremia, UTI, or pneumonia

• ●

Gram-negative bacillus

• ●

Possible red pigment

→ Think SERRATIA MARCESCENS

Exam Essentials

Genus: Serratia

Most clinically important species: Serratia marcescens

Organism: Gram-negative bacillus

Order: Enterobacterales

Metabolism: Facultatively anaerobic

Oxidase: Negative

Motility: Usually motile

Classic pigment: Prodigiosin → red/pink pigmentation

Important caveat: Not all clinical isolates are visibly pigmented

Distribution: Worldwide

Major epidemiologic setting: Healthcare-associated infection

Risk factors: Hospitalization, invasive procedures/devices, critical illness, and injection drug use

Major infections: Bacteremia, UTI, pneumonia, and intra-abdominal infection

Additional infection: Endocarditis, particularly associated historically with injection drug use

Diagnosis: Culture

Resistance issue: Chromosomal AmpC β-lactamase plus potential acquired resistance mechanisms

Treatment principle: Susceptibility-guided antimicrobial therapy

Source-listed options: Ciprofloxacin/other fluoroquinolones, third-generation cephalosporins, carbapenems, aztreonam, ureidopenicillins, aminoglycosides, ofloxacin, and TMP-SMX when active

Management principle: Antibiotic therapy + appropriate source control

Key clinical pearl: Serratia marcescens is the most clinically important Serratia species and is primarily a healthcare-associated Gram-negative pathogen causing catheter-associated bacteremia, UTI, pneumonia, and other invasive infections. Its classic microbiology clue is production of the red pigment prodigiosin, although many clinical isolates are nonpigmented. Because Serratia possesses intrinsic and acquired antimicrobial resistance mechanisms, definitive therapy should be guided by susceptibility testing rather than by a fixed antibiotic regimen.