A carbapenem-resistant Enterobacteriaceae (CRE) isolate is positive by the modified carbapenem inactivation method (mCIM) but negative by the EDTA-mCIM (eCIM). Which carbapenemase class is most consistent with this result?
- A KPC (class A serine carbapenemase) ✓
- B NDM (metallo-beta-lactamase, class B)
- C OXA-48-like (class D)
- D VIM (metallo-beta-lactamase, class B)
Correct answer: A. KPC (class A serine carbapenemase)
Explanation
The mCIM detects carbapenemase activity broadly; the eCIM adds EDTA, which chelates zinc and inhibits metallo-beta-lactamases (class B: NDM, VIM, IMP). A positive mCIM with negative eCIM indicates the carbapenemase is NOT a metallo-enzyme, pointing to a serine carbapenemase such as KPC (class A) or OXA-48 (class D). NDM and VIM would give a positive eCIM (inhibition by EDTA restores susceptibility). OXA-48 typically gives weakly positive mCIM but negative eCIM as well; however, KPC is the most commonly tested answer in this context.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.