An ICU patient develops a wound infection. Swab grows Staphylococcus aureus. Disc diffusion shows resistance to oxacillin; the isolate also tests positive by cefoxitin disc test. Real-time PCR of the isolate is positive for mecA gene. The mechanism underlying cefoxitin disc resistance is best explained by:
- A Acquisition of mecA gene encoding penicillin-binding protein 2a (PBP2a) with markedly reduced affinity for all beta-lactams ✓
- B Production of broad-spectrum metallo-beta-lactamase that hydrolyses cefoxitin
- C Overexpression of efflux pump MexAB-OprM expelling cefoxitin from the cell
- D Constitutive production of beta-lactamase encoded by blaZ gene that inactivates cefoxitin
Explanation
MRSA resistance is mediated by the mecA gene (located on SCCmec mobile genetic element) which encodes PBP2a (also called PBP2'). PBP2a has an extremely low binding affinity for all beta-lactam antibiotics, including cefoxitin. Standard PBPs (1–4) responsible for cell wall synthesis are saturated by beta-lactams at normal concentrations, but PBP2a continues transpeptidation, enabling growth. The cefoxitin disc test is used as a surrogate for oxacillin resistance because PBP2a detection is more reliable with cefoxitin. MexAB-OprM is an efflux pump found in Pseudomonas aeruginosa, not Staphylococcus. blaZ encodes penicillinase, which does not efficiently hydrolyse cefoxitin.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.