Which statement about penicillins and MRSA is true?

Which statement about penicillins and MRSA is true?

• A. Penicillins commonly cover MRSA
• B. Penicillins do not cover MRSA
• C. Penicillins cover MRSA when used with beta-lactamase inhibitors
• D. Penicillins cover MRSA in high dose

Answer: (B)

The key idea is that MRSA resists penicillins because of an altered target, not because of enzyme destruction. MRSA carries the mecA gene, which produces PBP2a, a penicillin-binding protein with very low affinity for beta-lactams. Since the drug can’t effectively bind its target, it cannot reliably block cell wall synthesis, so bacterial growth isn’t stopped. Adding beta-lactamase inhibitors won’t fix this, because the resistance isn’t due to beta-lactamase activity but to the target itself. Raising the dose doesn’t help either, because even high concentrations won’t overcome the very weak binding to PBP2a. Therefore, penicillins do not cover MRSA. For MRSA, other antibiotic classes—or newer agents with activity against PBP2a (not standard penicillins)—are needed.

The key idea is that MRSA resists penicillins because of an altered target, not because of enzyme destruction. MRSA carries the mecA gene, which produces PBP2a, a penicillin-binding protein with very low affinity for beta-lactams. Since the drug can’t effectively bind its target, it cannot reliably block cell wall synthesis, so bacterial growth isn’t stopped. Adding beta-lactamase inhibitors won’t fix this, because the resistance isn’t due to beta-lactamase activity but to the target itself. Raising the dose doesn’t help either, because even high concentrations won’t overcome the very weak binding to PBP2a. Therefore, penicillins do not cover MRSA. For MRSA, other antibiotic classes—or newer agents with activity against PBP2a (not standard penicillins)—are needed.