Staphylococcus aureus
🚨 Growth of Staphylococcus aureus in blood cultures should never be dismissed as a contaminant unless all causes are excluded and repeat cultures are negative. Always consider the possibility of endocarditis.
📖 About
- A common skin commensal with major pathogenic potential → causes mild to life-threatening infections.
- Produces numerous toxins (e.g. superantigens) that can trigger severe systemic immune responses.
🔬 Characteristics
- Gram-positive cocci in “grape-like” clusters.
- Aerobic or facultative anaerobe; salt-tolerant (grows on mannitol salt agar).
- 🧪 Coagulase-positive (key differentiator from coagulase-negative staphylococci).
- 🧪 Catalase positive.
- β-haemolytic on blood agar.
- Surface protein A binds Fc region of IgG → blocks opsonisation and phagocytosis.
⚔️ Virulence Factors
- 🧪 Coagulase → fibrin clot formation, immune evasion.
- 🧪 Staphylokinase → dissolves clots, aids spread.
- 🧪 Hyaluronidase → breaks down connective tissue.
- 🧪 Haemolysins → RBC lysis.
- ☠️ PVL (Panton-Valentine leucocidin) → WBC destruction, aggressive skin infections.
- ⚡ TSST-1 → superantigen, causes toxic shock syndrome.
- 🔥 Exfoliative toxins → scalded skin syndrome.
- 🛡️ Capsule → prevents phagocytosis.
🏠 Source
- Carried in the nasal mucosa, skin, and moist body sites.
- Risk groups: healthcare workers, IV drug users, diabetics, immunocompromised patients.
⚠️ Pathogenicity
- 🍗 Food poisoning: Enterotoxins → rapid nausea, vomiting, abdominal pain (1–6 h post-ingestion).
- 🩹 Skin/soft tissue infections: Impetigo, boils, folliculitis, cellulitis, abscesses; toxins → scalded skin syndrome (Ritter’s).
- 🚨 Toxic Shock Syndrome (TSS): Classically with tampon use or wound packing; TSST-1 triggers cytokine storm → shock, multiorgan failure.
- ❤️ Endocarditis: Especially in IV drug users, prosthetic valves, indwelling IV catheters.
- 🦴 Osteomyelitis & 🫁 Pneumonia: Post-influenza pneumonia is classic.
🔎 Investigations
- Coagulase test: Positive.
- DNAse test: Positive (unlike CoNS).
- Mannitol salt agar: Grows well, golden-yellow colonies.
- Blood culture: Always significant unless proven otherwise.
- PCR/phage typing: for epidemiology/strain ID.
💊 Management
- MSSA (Methicillin-sensitive S. aureus):
- First-line: Flucloxacillin (UK). Cephalosporins or clindamycin if allergy.
- Other options: Fusidic acid (esp. skin infections), vancomycin if severe.
- MRSA (Methicillin-resistant S. aureus):
- Resistance due to mecA gene → altered PBP2a.
- First-line: IV vancomycin or teicoplanin.
- Alternatives: Linezolid, daptomycin (severe/systemic infections).
🧪 Sensitivity
- MSSA → sensitive to β-lactams (flucloxacillin, nafcillin).
- MRSA → requires glycopeptides (vancomycin/teicoplanin) or newer agents.
