If a blood culture grows MSSA, what is the recommended antibiotic change?

• A. Continue vancomycin
• B. Switch to nafcillin
• C. Use amoxicillin
• D. Use doxycycline

Answer: (B)

When MSSA is identified in a blood culture, switch to a beta-lactamase–resistant penicillin such as nafcillin. MSSA is highly susceptible to these agents, which provide rapid, reliable bactericidal activity in bloodstream infections. Vancomycin remains useful mainly for MRSA or when a patient cannot receive beta-lactams due to allergy, but it is not as effective against MSSA and can have slower killing. Amoxicillin would be inactivated by the penicillinase produced by Staphylococcus aureus, so it’s not reliable for MSSA bacteremia. Doxycycline is not preferred for bacteremia because its pharmacodynamics are less favorable for bloodstream infections.