Staphylococcus aureus
๐จ 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.
๐ About
- A common skin commensal with major pathogenic potential โ causes mild to life-threatening infections.
- Produces numerous toxins (e.g. superantigens) that can trigger severe systemic immune responses.
๐ฌ Characteristics
- Gram-positive cocci in โgrape-likeโ clusters.
- Aerobic or facultative anaerobe; salt-tolerant (grows on mannitol salt agar).
- ๐งช Coagulase-positive (key differentiator from coagulase-negative staphylococci).
- ๐งช Catalase positive.
- ฮฒ-haemolytic on blood agar.
- Surface protein A binds Fc region of IgG โ blocks opsonisation and phagocytosis.
โ๏ธ Virulence Factors
- ๐งช Coagulase โ fibrin clot formation, immune evasion.
- ๐งช Staphylokinase โ dissolves clots, aids spread.
- ๐งช Hyaluronidase โ breaks down connective tissue.
- ๐งช Haemolysins โ RBC lysis.
- โ ๏ธ PVL (Panton-Valentine leucocidin) โ WBC destruction, aggressive skin infections.
- โก TSST-1 โ superantigen, causes toxic shock syndrome.
- ๐ฅ Exfoliative toxins โ scalded skin syndrome.
- ๐ก๏ธ Capsule โ prevents phagocytosis.
๐ Source
- Carried in the nasal mucosa, skin, and moist body sites.
- Risk groups: healthcare workers, IV drug users, diabetics, immunocompromised patients.
โ ๏ธ Pathogenicity
- ๐ Food poisoning: Enterotoxins โ rapid nausea, vomiting, abdominal pain (1โ6 h post-ingestion).
- ๐ฉน Skin/soft tissue infections: Impetigo, boils, folliculitis, cellulitis, abscesses; toxins โ scalded skin syndrome (Ritterโs).
- ๐จ Toxic Shock Syndrome (TSS): Classically with tampon use or wound packing; TSST-1 triggers cytokine storm โ shock, multiorgan failure.
- โค๏ธ Endocarditis: Especially in IV drug users, prosthetic valves, indwelling IV catheters.
- ๐ฆด Osteomyelitis & ๐ซ Pneumonia: Post-influenza pneumonia is classic.
๐ 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.
๐ 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 โ altered PBP2a.
- First-line: IV vancomycin or teicoplanin.
- Alternatives: Linezolid, daptomycin (severe/systemic infections).
๐งช Sensitivity
- MSSA โ sensitive to ฮฒ-lactams (flucloxacillin, nafcillin).
- MRSA โ requires glycopeptides (vancomycin/teicoplanin) or newer agents.
