Staphylococcus aureus

Quick Profile

• Classification: Gram-positive cocci in clusters
• Key labs: Catalase positive, coagulase positive, beta-hemolytic, mannitol fermenting
• Colonization: Skin and anterior nares
• Clinical style: Both toxin-mediated disease and invasive pyogenic infection

Identification And Classification

• Organism family: Staphylococci
• Distinguishing point from streptococci: Catalase positive
• Distinguishing point from Staphylococcus epidermidis: Coagulase positive

Reservoir And Transmission

• Human reservoir, especially skin and nasal carriers
• Spread by direct contact or fomites
• Risk increases with skin breaks, indwelling devices, postoperative state, and IV drug use

Virulence Factors And Pathogenesis

• Protein A binds Fc region of IgG, reducing opsonization
• Coagulase promotes fibrin clot formation around bacteria
• Cytotoxins and leukocidins contribute to tissue destruction
• Exfoliative toxins split epidermal layers (scalded skin syndrome)
• Enterotoxins are preformed and heat stable (rapid-onset food poisoning)
• TSST-1 superantigen drives toxic shock syndrome

High-Yield Disease Patterns

Skin And Soft Tissue

• Cellulitis, abscesses, furuncles, carbuncles, impetigo
• Hallmark clue: Purulent, localized infections with abscess formation

Toxin-Mediated Syndromes

• Food poisoning: rapid vomiting after mayonnaise, cream, or meat exposure
• Toxic shock syndrome: fever, hypotension, diffuse rash, multi-organ involvement
• Staphylococcal scalded skin syndrome: epidermal peeling in infants and children

Invasive Disease

• Pneumonia, especially post-influenza necrotizing pneumonia
• Acute endocarditis (can affect normal valves in IV drug use)
• Osteomyelitis and septic arthritis
• Bacteremia with metastatic abscesses

Diagnosis

• Gram stain and culture from the suspected source
• Blood cultures for systemic disease
• Echo for endocarditis evaluation when bacteremia is persistent or high risk
• Susceptibility testing to determine MSSA versus MRSA

Treatment And Resistance

MSSA

• Preferred therapy: anti-staphylococcal beta-lactams (for example nafcillin, oxacillin, cefazolin)

MRSA

• Common mechanism: altered penicillin-binding protein (PBP2a) from mecA gene
• Severe disease options: vancomycin, daptomycin (not for pneumonia), linezolid
• Community skin infection options may include TMP-SMX, doxycycline, or clindamycin based on local resistance

For antimicrobial classes and mechanisms, see Pharmacology: Antimicrobials.

Exam Traps

• Staph aureus food poisoning is caused by preformed toxin, so incubation is short and fever may be absent
• Daptomycin is inactivated by lung surfactant and should not be used for pneumonia
• Do not confuse Staphylococcus aureus (coagulase positive) with Staphylococcus epidermidis (coagulase negative)