The antibacterial activity of carbapenems against MRSA is limited despite their broad-spectrum beta-lactam activity. What is the molecular basis?
- A MRSA produces metallo-beta-lactamase that hydrolyzes carbapenems
- B Carbapenems cannot penetrate the outer membrane of MRSA
- C MRSA produces modified penicillin-binding protein PBP2a (encoded by mecA gene) with low affinity for all beta-lactams including carbapenems ✓
- D MRSA efflux pumps actively remove carbapenems from the bacterial cell
Correct answer: C. MRSA produces modified penicillin-binding protein PBP2a (encoded by mecA gene) with low affinity for all beta-lactams including carbapenems
Explanation
MRSA resistance is mediated by the mecA gene (or the newer mecC gene), which encodes PBP2a, an altered penicillin-binding protein with extremely low affinity for all beta-lactam antibiotics including carbapenems. Because PBP2a can substitute for the normal transpeptidase function even when other PBPs are inhibited, cell wall synthesis continues and the bacteria survive. This is a target modification mechanism, fundamentally different from enzymatic hydrolysis. Metallo-beta-lactamases (which do hydrolyze carbapenems) are found in gram-negative organisms, not MRSA.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.