A clinical microbiology lab reports a Klebsiella pneumoniae blood culture isolate as carbapenem resistant. Meropenem MIC is 4 µg/mL (resistant). Modified carbapenem inactivation method (mCIM) is positive. Phenotypic testing with EDTA-mCIM is also positive. Which enzyme class is most likely responsible?
- A KPC (Klebsiella pneumoniae carbapenemase) — class A serine carbapenemase
- B Metallo-beta-lactamase (MBL — NDM-1 type) ✓
- C OXA-48 — class D carbapenemase
- D Loss of OmpK36 porin with AmpC overexpression
Correct answer: B. Metallo-beta-lactamase (MBL — NDM-1 type)
Explanation
The modified carbapenem inactivation method (mCIM) combined with EDTA-mCIM differentiates MBL from serine carbapenemases (KPC, OXA-48): a positive mCIM with positive EDTA mCIM inhibition indicates MBL activity because EDTA chelates zinc ions required by metallo-enzymes (NDM, VIM, IMP). KPC gives a positive mCIM but negative EDTA-mCIM (not inhibited by EDTA). OXA-48 may give weakly positive mCIM, EDTA-negative. NDM-1-producing K. pneumoniae is the dominant carbapenemase in India per ICMR surveillance data.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.