## Methicillin-Resistant S. aureus (MRSA) Endocarditis — First-Line Therapy **Key Point:** For MRSA endocarditis, **vancomycin** is the drug of choice. It is the only glycopeptide with proven efficacy in MRSA bacteremia and endocarditis, achieving adequate cardiac vegetative penetration and bactericidal activity [cite:Harrison 21e Ch 297]. ### Mechanism of Action Vancomycin is a glycopeptide antibiotic that inhibits bacterial cell wall synthesis by binding to D-Ala–D-Ala residues of peptidoglycan precursors. It is bactericidal against MRSA and achieves therapeutic levels in vegetations. ### Vancomycin Dosing for Endocarditis 1. **IV vancomycin:** 15–20 mg/kg every 8–12 hours (target trough: 15–20 µg/mL) 2. **Duration:** Minimum 4 weeks for native valve endocarditis; 6 weeks for prosthetic valve 3. **Monitoring:** Baseline and periodic renal function, vancomycin trough levels (day 3–5) ### Why Vancomycin is Preferred in MRSA Endocarditis - **Methicillin resistance** = altered PBPs → β-lactams ineffective - **Vancomycin penetration** into cardiac vegetations is adequate (though slower than β-lactams) - **Bactericidal activity** confirmed in endocarditis models and clinical trials - **No viable alternatives** with equivalent efficacy for MRSA (linezolid is bacteriostatic) **High-Yield:** Daptomycin (5 mg/kg IV daily) is an emerging alternative for MRSA endocarditis but is NOT first-line in most guidelines because vancomycin remains the gold standard [cite:KD Tripathi 8e Ch 12]. ### Clinical Pearl MRSA is typically acquired in healthcare settings (HA-MRSA) or the community (CA-MRSA). IV drug users have high risk of MRSA endocarditis due to skin flora seeding. Always obtain vancomycin trough levels to ensure adequate dosing and minimize nephrotoxicity. **Mnemonic:** **MRSA = Vancomycin (V)** — when you see MRSA, think vancomycin first.
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