Makindo Medical Notes

🚨 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.