First-line antibiotics for Methicillin-susceptible Staphylococcus aureus (MSSA) infections include which of the following?

First-line antibiotics for Methicillin-susceptible Staphylococcus aureus (MSSA) infections include which of the following?

• A. Penicillinase-resistant penicillins such as nafcillin
• B. Vancomycin
• C. Penicillin G
• D. Azithromycin

Answer: (A)

MSSA infections are best treated with penicillinase-resistant penicillins because Staphylococcus aureus commonly makes penicillinase, which hydrolyzes ordinary penicillins like penicillin G. The bulky side chains of drugs such as nafcillin (and related agents like oxacillin or dicloxacillin) protect the beta-lactam ring from that enzyme, so these drugs stay effective against MSSA. They’re bactericidal and reliably cover MSSA infections, making them the preferred first-line option. Vancomycin is used mainly for MRSA or when a patient cannot receive beta-lactams, not as the first choice for MSSA. Penicillin G is inactivated by penicillinase, so it isn’t effective against MSSA. Azithromycin has limited activity against staphylococci and isn’t a preferred agent for MSSA infections.

MSSA infections are best treated with penicillinase-resistant penicillins because Staphylococcus aureus commonly makes penicillinase, which hydrolyzes ordinary penicillins like penicillin G. The bulky side chains of drugs such as nafcillin (and related agents like oxacillin or dicloxacillin) protect the beta-lactam ring from that enzyme, so these drugs stay effective against MSSA. They’re bactericidal and reliably cover MSSA infections, making them the preferred first-line option.

Vancomycin is used mainly for MRSA or when a patient cannot receive beta-lactams, not as the first choice for MSSA. Penicillin G is inactivated by penicillinase, so it isn’t effective against MSSA. Azithromycin has limited activity against staphylococci and isn’t a preferred agent for MSSA infections.