Infectious Disease and Microbiology – Staphylococcus Species

Overview

Staphylococcus species are Gram-positive cocci that are major components of normal human skin and mucosal flora but also rank among the most important causes of human bacterial infection. They can infect almost every body site and are particularly important causes of skin and soft-tissue infection, abscesses, bacteremia, endocarditis, pneumonia, osteomyelitis, septic arthritis, surgical-site infection, and medical device-associated infection.

The two most important clinical organisms are Staphylococcus aureus and Staphylococcus epidermidis. S. aureus is distinguished by its coagulase positivity and aggressive virulence, whereas most other clinically important staphylococci are coagulase-negative staphylococci (CoNS) that frequently cause infections involving prosthetic material and intravascular devices.

Classification

Important species include:

• Staphylococcus aureus

• S. capitis

• S. cohnii

• S. epidermidis

• S. gallinarum

• S. haemolyticus

• S. hominis

• S. intermedius

• S. lugdunensis

• S. warneri

• S. xylosus

• Other Staphylococcus species

Microbiologic Characteristics

Staphylococci are:

• Gram-positive cocci

• Facultatively anaerobic

• Nonmotile

• Non-spore-forming

• Catalase positive

• Typically arranged in irregular grape-like clusters

The name Staphylococcus comes from the characteristic clustered appearance.

High-Yield Microbiology Pattern

Gram-positive cocci

• ●

Grape-like clusters

• ●

Catalase positive

→ Think STAPHYLOCOCCUS

Coagulase Classification

A major clinical division separates staphylococci into:

Coagulase-Positive Staphylococci

Most importantly:

Staphylococcus aureus

Coagulase-Negative Staphylococci

Includes many species, particularly:

S. epidermidis

as well as S. haemolyticus, S. hominis, and others.

High-Yield Identification

Catalase-positive GPC

↓

Perform coagulase test

Coagulase positive

→ Think S. aureus

Coagulase negative

→ Think CoNS, especially S. epidermidis

Epidemiology

Staphylococci have a:

Worldwide distribution

Many species normally colonize:

• Skin

• Anterior nares

• Oropharynx

• Other mucosal surfaces

Colonization provides a reservoir from which endogenous infections can develop when normal barriers are disrupted.

Incubation Period

The incubation period is:

Highly variable

because Staphylococcus causes many different diseases through different mechanisms.

For example, disease may result from:

Direct tissue invasion

or

Preformed bacterial toxins

so there is no single incubation period applicable to the genus.

Staphylococcus aureus

S. aureus is the most important and virulent human staphylococcal pathogen.

It characteristically causes:

SUPPURATIVE INFECTIONS

with a strong tendency toward:

ABSCESS FORMATION

Classic S. aureus Laboratory Features

S. aureus is classically:

Gram-positive

• ●

Catalase positive

• ●

Coagulase positive

• ●

Often β-hemolytic

• ●

Often produces golden-yellow colonies

The species name aureus refers to its characteristic:

Golden pigmentation

Abscess Formation

A hallmark of S. aureus infection is:

Localized pus and abscess formation

Examples include:

• Furuncles

• Carbuncles

• Skin abscesses

• Deep-tissue abscesses

• Organ abscesses

High-Yield Clinical Pattern

Painful purulent skin lesion

• ●

Abscess

• ●

Gram-positive cocci in clusters

• ●

Coagulase positive

→ Think STAPHYLOCOCCUS AUREUS

Skin and Soft-Tissue Infections

S. aureus is a major cause of:

• Folliculitis

• Furuncles

• Carbuncles

• Abscesses

• Cellulitis

• Wound infections

• Surgical-site infections

Purulence strongly suggests:

Staphylococcal infection, particularly S. aureus.

Bacteremia

S. aureus is an important cause of:

BACTEREMIA

Potential sources include:

• Skin and soft tissue

• Central venous catheters

• Endocarditis

• Pneumonia

• Bone and joint infection

• Surgical wounds

S. aureus bacteremia is clinically important because hematogenous spread may seed distant organs.

Metastatic Infection

Bloodstream dissemination can produce:

Endocarditis

Osteomyelitis

Septic arthritis

Epidural abscess

Deep-organ abscesses

and other metastatic infections.

Therefore, clinically significant S. aureus in blood should generally be treated as a:

True pathogen rather than a contaminant.

Endocarditis

Staphylococci are major causes of:

INFECTIVE ENDOCARDITIS

S. aureus can produce aggressive infection of:

Native or prosthetic valves

and is particularly associated with endocarditis in:

People who inject drugs

and patients with healthcare-associated bloodstream infection.

High-Yield Endocarditis Pattern

Injection drug use

• ●

Acute fever

• ●

Bacteremia

• ●

Tricuspid valve vegetation

→ Think S. aureus

Osteomyelitis

S. aureus is a major cause of:

Osteomyelitis

Infection may arise through:

Hematogenous spread

or

Direct inoculation/contiguous extension

Septic Arthritis

S. aureus is also a major cause of:

Acute bacterial septic arthritis

Typical presentation includes:

Painful + swollen + warm joint with restricted movement

Pneumonia

S. aureus can cause:

Severe pneumonia

including healthcare-associated disease and pneumonia following:

Influenza or another viral respiratory infection

Necrotizing or cavitary disease can occur.

Toxin-Mediated Disease

S. aureus can cause illness not only by direct tissue invasion but also through:

TOXIN PRODUCTION

Important toxin-mediated syndromes include:

• Food poisoning

• Toxic shock syndrome

• Staphylococcal scalded skin syndrome

Staphylococcal Food Poisoning

Food poisoning results from ingestion of:

Preformed, heat-stable enterotoxin

The classic presentation is:

Rapid-onset nausea and prominent vomiting

after consumption of contaminated food.

Because toxin is already present in the food, the incubation period is:

Short

High-Yield Food Poisoning Pattern

Food exposure

• ●

Very rapid onset

• ●

Prominent vomiting

• ●

Preformed toxin

→ S. aureus food poisoning

Toxic Shock Syndrome

Toxic shock syndrome results from bacterial toxins acting as:

Superantigens

Clinical manifestations can include:

High fever + hypotension + diffuse rash + multiorgan dysfunction

Staphylococcal Scalded Skin Syndrome

Certain S. aureus strains produce:

Exfoliative toxins

which disrupt epidermal adhesion.

This can produce:

Tender erythema + superficial blistering + widespread desquamation

particularly in infants and young children.

Staphylococcus epidermidis

S. epidermidis is one of the most clinically important:

Coagulase-negative staphylococci

It is a common member of:

Normal skin flora

and is therefore also a frequent:

Blood-culture contaminant

However, it is an important true pathogen when:

Foreign material or implanted medical devices are present.

Biofilm Formation

A key virulence mechanism of S. epidermidis is:

BIOFILM FORMATION

Biofilm allows organisms to adhere to artificial surfaces and resist:

Host immune defenses + antimicrobial therapy

Device-Associated Infection

S. epidermidis is particularly associated with:

• Central venous catheters

• Prosthetic heart valves

• Prosthetic joints

• CSF shunts

• Pacemakers and other implanted devices

High-Yield S. epidermidis Pattern

Prosthetic material

• ●

Indolent infection

• ●

Coagulase-negative staphylococcus

• ●

Biofilm

→ Think STAPHYLOCOCCUS EPIDERMIDIS

Central Venous Catheters

The increased use of:

Central venous catheters

has contributed significantly to the importance of coagulase-negative staphylococci as causes of:

Healthcare-associated bloodstream infection

Skin flora can gain access to the catheter and establish a:

Biofilm-associated infection

Staphylococcus lugdunensis

S. lugdunensis is a particularly important exception among coagulase-negative staphylococci.

Although technically a:

Coagulase-negative staphylococcus

its clinical behavior may be considerably more aggressive and resemble:

S. aureus

It is particularly important as a cause of:

Infective endocarditis

and significant skin/soft-tissue infection.

High-Yield Exception

Coagulase-negative

BUT

Aggressive infection similar to S. aureus

→ Think S. LUGDUNENSIS

Diagnosis

The principal diagnostic method is:

CULTURE

Appropriate specimens include:

• Blood

• Abscess material

• Wound specimens

• Synovial fluid

• Bone specimens

• Respiratory specimens

• CSF

• Other normally sterile fluids or tissues

Gram Stain

Typical microscopy demonstrates:

Gram-positive cocci arranged in clusters

This provides an early clue before definitive culture identification.

Catalase Test

The catalase test distinguishes:

Staphylococcus

from:

Streptococcus/Enterococcus

Staphylococcus

Catalase positive

Streptococcus/Enterococcus

Catalase negative

Classic Exam Pattern

GPC in clusters + catalase positive

→ Staphylococcus

GPC in chains/pairs + catalase negative

→ Streptococcus/Enterococcus

Coagulase Test

The coagulase test helps identify:

S. aureus

Coagulase positive

→ S. aureus

Coagulase negative

→ Most other clinically important staphylococci

Methicillin-Susceptible S. aureus

Methicillin-susceptible S. aureus is abbreviated:

MSSA

For serious MSSA infections, preferred β-lactams traditionally include:

Nafcillin

or

Oxacillin

with other appropriate antistaphylococcal β-lactams used depending on the clinical setting.

Methicillin-Resistant S. aureus

Methicillin-resistant S. aureus is abbreviated:

MRSA

Resistance is most commonly mediated through an altered penicillin-binding protein:

PBP2a

encoded by:

mecA

or related resistance determinants.

High-Yield MRSA Mechanism

mecA

↓

PBP2a

↓

Reduced affinity for β-lactam antibiotics

↓

Methicillin resistance

→ MRSA

Healthcare-Associated MRSA

MRSA is particularly important in:

• Hospitals

• Long-term care facilities

• Dialysis populations

• Patients with invasive devices

• Patients with repeated healthcare exposure

Community-Associated MRSA

MRSA can also cause community-associated disease, particularly:

Purulent skin and soft-tissue infections

including:

Abscesses

Treatment of MSSA

The source identifies:

Antistaphylococcal penicillins

such as:

• Oxacillin

• Nafcillin

• Cloxacillin

as preferred treatments for methicillin-susceptible strains.

For serious MSSA disease, an active anti-staphylococcal β-lactam is generally preferred over vancomycin when the patient can receive one.

Treatment of MRSA

For serious MRSA infections, the source lists:

Vancomycin

as an important treatment.

The exact antimicrobial regimen depends on:

Site of infection + disease severity + susceptibility + patient factors

Coagulase-Negative Staphylococci

A significant proportion of clinically important CoNS are:

Methicillin resistant

Therefore, serious device-associated infection may require:

Vancomycin or another appropriately active agent

while susceptibility results are being considered.

Oral Treatment Options

The source notes that susceptible isolates may respond to:

• Doxycycline

• Clindamycin

• Trimethoprim–sulfamethoxazole

These agents are particularly relevant to selected:

MRSA skin and soft-tissue infections

but are not interchangeable for every invasive staphylococcal syndrome.

Abscess Treatment

For a drainable staphylococcal abscess, a fundamental treatment principle is:

INCISION AND DRAINAGE

Antibiotics alone may not adequately treat a large collection of purulent material.

High-Yield Abscess Management

S. aureus abscess

→ Incision and drainage

• ●

Antimicrobial therapy when clinically indicated

Device-Associated Infection

For infections involving:

Central lines, prosthetic joints, shunts, or other foreign material

successful treatment may require:

Antibiotic therapy

• ●

Removal or revision of the infected device when appropriate

because biofilm can make eradication difficult.

Staphylococcus vs. Streptococcus

Staphylococcus

→ Gram-positive cocci

→ Clusters

→ Catalase positive

Streptococcus

→ Gram-positive cocci

→ Usually chains or pairs

→ Catalase negative

S. aureus vs. S. epidermidis

S. aureus

Coagulase: Positive

Virulence: High

Classic disease: Abscesses and invasive infection

Toxin-mediated disease: Common

S. epidermidis

Coagulase: Negative

Virulence: Lower

Classic disease: Prosthetic/device-associated infection

Major mechanism: Biofilm

High-Yield Memory Pattern

AUREUS = ABSCESS

S. aureus

→ Coagulase positive

→ Abscess formation

EPIDERMIDIS = EQUIPMENT

S. epidermidis

→ Coagulase negative

→ Biofilm

→ Prosthetic equipment/device infection

Prevention

Prevention includes:

• Hand hygiene

• Appropriate wound care

• Proper central-line insertion and maintenance

• Removal of unnecessary invasive devices

• Appropriate surgical-site infection prevention

• Infection-control measures for resistant organisms

• Antimicrobial stewardship

High-Yield Clinical Pattern

Gram-positive cocci in clusters

• ●

Catalase positive

• ●

Coagulase positive

• ●

Purulent abscess

→ Think STAPHYLOCOCCUS AUREUS

Coagulase-negative staphylococcus

• ●

Central venous catheter/prosthetic device

• ●

Biofilm-associated bacteremia

→ Think STAPHYLOCOCCUS EPIDERMIDIS

Coagulase-negative

• ●

Unexpectedly aggressive endocarditis

→ Think STAPHYLOCOCCUS LUGDUNENSIS

Exam Essentials

Genus: Staphylococcus

Organism: Gram-positive cocci

Arrangement: Grape-like clusters

Catalase: Positive

Distribution: Worldwide

Incubation: Highly variable

Major coagulase-positive species: S. aureus

Classic S. aureus infection: Abscess/purulent infection

Other major S. aureus diseases: Bacteremia, endocarditis, pneumonia, osteomyelitis, septic arthritis, and toxin-mediated syndromes

MSSA: Methicillin-susceptible S. aureus

MRSA: Methicillin-resistant S. aureus

Classic MRSA mechanism: mecA → PBP2a

Major coagulase-negative species: S. epidermidis

Classic S. epidermidis association: Prosthetic material and intravascular catheters

Major virulence mechanism: Biofilm formation

Important CoNS exception: S. lugdunensis can cause aggressive disease, particularly endocarditis

Diagnosis: Culture

MSSA treatment principle: Anti-staphylococcal β-lactam such as nafcillin or oxacillin when appropriate

MRSA treatment principle: Site- and susceptibility-directed therapy; vancomycin remains an important option for serious disease

Selected susceptible skin isolates: Doxycycline, clindamycin, or TMP-SMX may be useful

Abscess management: Incision and drainage

Device infection: Consider device removal/source control

Key clinical pearl: Think of Staphylococcus when Gram-positive cocci occur in grape-like clusters and are catalase positive. S. aureus is coagulase positive and classically causes aggressive purulent disease and abscesses, whereas S. epidermidis is coagulase negative and specializes in biofilm-associated infections of central lines and prosthetic devices. S. lugdunensis is the important coagulase-negative exception because it can behave aggressively like S. aureus.