## Drug of Choice for Streptococcus viridans Endocarditis **Key Point:** Penicillin G (high-dose IV) combined with gentamicin is the gold standard first-line regimen for native valve endocarditis caused by penicillin-susceptible streptococci (including S. viridans). ### Rationale for Penicillin G + Gentamicin 1. **Penicillin G** — bactericidal against streptococci; achieves excellent intracardiac penetration at high doses (18–24 million units/day IV in divided doses) 2. **Gentamicin** — synergistic aminoglycoside that enhances bactericidal activity and shortens duration of bacteremia; typically dosed at 3 mg/kg/day IV in divided doses 3. **Duration** — 4 weeks for native valve, 6 weeks for prosthetic valve (in susceptible strains) ### Comparison of Regimens | Agent | Indication | Notes | |-------|-----------|-------| | **Penicillin G + Gentamicin** | Penicillin-susceptible streptococci (native valve) | First-line; synergistic; 4-week course | | Ceftriaxone | Alternative if β-lactam allergy (non-anaphylaxis) | Less preferred than penicillin for streptococci | | Vancomycin + Rifampicin | MRSA, prosthetic valve, or penicillin allergy (anaphylaxis) | Reserved for resistant organisms or allergy | | Fluoroquinolones | Oral step-down therapy only; NOT monotherapy | Poor intracardiac penetration; inadequate for acute IE | **High-Yield:** The combination of penicillin + gentamicin is **synergistic** — gentamicin enhances penicillin's bactericidal effect and is critical for rapid sterilization of vegetations. **Clinical Pearl:** S. viridans is an alpha-hemolytic streptococcus commonly found in the oral flora and is the most frequent cause of subacute endocarditis in patients with underlying valvular disease (e.g., rheumatic heart disease). **Warning:** Do NOT use fluoroquinolones or cephalosporins as monotherapy for acute IE — they lack the synergy needed for rapid cure and have unacceptably high failure rates.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.