A clinical laboratory tests a Klebsiella pneumoniae isolate from a catheter-associated urinary tract infection. The disk diffusion shows resistance to ertapenem but susceptibility to imipenem. Phenotypic testing with meropenem + EDTA reveals synergy. Which mechanism best explains this pattern?
- A KPC (class A carbapenemase) production
- B MBL (metallo-beta-lactamase) production (VIM/NDM type) ✓
- C OXA-48-type carbapenemase production
- D Loss of outer membrane porin OmpK36 alone
Correct answer: B. MBL (metallo-beta-lactamase) production (VIM/NDM type)
Explanation
Metallo-beta-lactamases (MBLs) such as VIM and NDM are zinc-dependent enzymes inhibited by EDTA; meropenem + EDTA synergy (MHT negative, CIM/eCIM positive) is a phenotypic hallmark. The variable ertapenem-resistant but imipenem-susceptible pattern occurs because ertapenem has lower intrinsic stability to MBLs. KPC (class A) is inhibited by boronic acid, not EDTA; OXA-48 shows different inhibitor profiles; porin loss alone typically causes low-level resistance uniformly across carbapenems.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.