🫀🦠 MSSA Endocarditis

A 48‑year‑old man with poorly controlled type 2 diabetes and a history of intravenous heroin use presents with three days of fever, rigors, and pleuritic chest pain.

• Vitals: T 39.2 °C, HR 110 bpm, BP 118/66 mm Hg
• Exam: new holosystolic murmur at the left lower sternal border; several Janeway lesions on the palms; no focal neurological deficits.
• Investigations: Three blood‑culture sets grow methicillin‑susceptible Staphylococcus aureus (MSSA) within ten hours (oxacillin MIC ≤ 2 µg/mL). Transthoracic echo shows a 1.2 cm vegetation on the tricuspid valve with mild regurgitation. Renal and liver function are normal.
He has no β‑lactam allergy.

QUESTION
Which intravenous antibiotic regimen is most appropriate to start—and continue—for this infection?

A) Cefazolin 2 g IV every 8 h for at least six weeks
B) Vancomycin dosed to trough 15–20 mg/L for at least six weeks
C) Daptomycin 10 mg/kg IV once daily for at least six weeks
D) Linezolid 600 mg IV every 12 h for at least six weeks
E) Gentamicin 3 mg/kg IV once daily plus rifampicin 300 mg orally every 8 h

CORRECT ANSWER
A) Cefazolin 2 g IV every 8 h for at least six weeks

WHY OPTION A IS BEST
‑ First‑line β‑lactam therapy for native‑valve MSSA endocarditis (AHA/ESC guidelines).
‑ Rapidly bactericidal, clears bacteraemia faster, and is associated with lower mortality than vancomycin in MSSA infections.
‑ Excellent penetration into right‑sided vegetations.

WHY THE OTHER OPTIONS ARE INFERIOR
‑ B) Vancomycin: Acceptable only when the patient has a severe β‑lactam allergy or MRSA. For MSSA, outcomes are consistently worse than with β‑lactams.
‑ C) Daptomycin: Effective but reserved for β‑lactam‑allergic or persistent MRSA cases; no advantage over cefazolin.
‑ D) Linezolid: Bacteriostatic against staphylococci, carries haematological toxicity with prolonged use, and is not guideline‑endorsed for native‑valve MSSA IE.
‑ E) Gentamicin + rifampicin: Aminoglycoside adds nephro/ototoxicity without outcome benefit in native‑valve staphylococcal IE; rifampicin is reserved for prosthetic material or hardware infections.

KEY TAKE‑HOME POINTS

1. For MSSA endocarditis, “β‑lactam beats vancomycin.”

2. Right‑sided uncomplicated MSSA IE in injection‑drug users can occasionally be treated for two weeks, but most patients still require a six‑week course—adjust once clinical stability and repeat imaging permit.

3. Avoid routine gentamicin for native‑valve staph IE; toxicity > benefit.

4. Reserve rifampicin for prosthetic valves or intracardiac hardware.

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