Why are beta-lactams preferred over vancomycin for MSSA endocarditis?

• A. Vancomycin is more effective
• B. Beta-lactams cause more side effects
• C. Beta-lactams are more efficacious than vancomycin
• D. Doxycycline is superior

Answer: (C)

The key idea is that, for infections caused by methicillin-susceptible Staphylococcus aureus, beta-lactam antibiotics provide more reliable and rapid bacterial killing than vancomycin. Beta-lactams such as nafcillin, oxacillin, or cefazolin directly inhibit cell wall synthesis and achieve higher kill rates against MSSA, leading to better clinical outcomes in endocarditis. Vancomycin kills MSSA more slowly and is associated with higher failure rates in endocarditis, along with nephrotoxicity and dosing challenges. Because MSSA is susceptible to beta-lactams, these drugs are preferred to maximize efficacy. Doxycycline is not suitable for MSSA endocarditis, and the idea that beta-lactams cause more side effects is not the reason they’re chosen—the main driver is superior efficacy.