A patient with fever and a new onset heart murmur has chronic mitral valve prolapse and blood cultures positive for group B streptococci. Which antibiotic therapy is most appropriate?

• A. Penicillin and gentamicin
• B. Levofloxacin and rifampin
• C. Vancomycin
• D. Amoxicillin-clavulanate

Answer: (A)

Infective endocarditis on a native mitral valve due to penicillin-susceptible group B streptococci is best treated with a penicillin-based regimen plus an aminoglycoside for synergy. Penicillin G rapidly kills the bacteria, and adding gentamicin enhances bactericidal activity within the vegetations by providing a secondary mechanism of action and improving penetration. This synergistic combo leads to more effective sterilization of the infected valve and a better outcome, often in a shorter course for susceptible streptococci. Other options are not ideal: vancomycin alone is broader and used mainly for penicillin allergy or resistant organisms; amoxicillin-clavulanate is an oral regimen not suitable for endocarditis; fluoroquinolone with rifampin is not standard initial therapy for native-valve streptococcal endocarditis and rifampin is mainly reserved for prosthetic valve infections.