USMLE (Fach) / Microbiology (Lektion)

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Gram-positive, β-hemolytic, catalase ⊕, coagulase ⊕, cocci in clusters.- Protein A (virulence factor) binds Fc-IgG, inhibiting complement activation and phagocytosis.- Forms fibrin clot around self → abscess.- Commonly colonizes the nares, ears, axilla, and groin.

Causes:- Inflammatory disease – skin infections, organ abscesses, pneumonia (often after influenza virus infection), endocarditis, septic arthritis, osteomyelitis.- Toxin-mediated – Toxic shock syndrome (TSST-1, superantigen), scalded skin syndrome (exfoliative toxin), rapid-onset food poisoning (enterotoxins).- MRSA (methicillin-resistant S aureus) infection – important cause of serious nosocomial and community-acquired infection; resistant to methicillin and nafcillin because of altered penicillin-binding protein.

Staphylococcal toxic shock syndrome (TSS) – fever, vomiting, rash, desquamation, shock, end-organ failure. ↑ AST, ↑ ALT, ↑ bilirubin. Associated with prolonged use of vaginal tampons or nasal packing. S aureus food poisoning due to ingestion of preformed toxin → short incubation period (2-6 hr) followed by nonbloody diarrhea and emesis. Enterotoxin is heat stable → not destroyed by cooking.

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