A Klebsiella pneumoniae blood culture isolate shows: MIC to imipenem >32 µg/mL, MIC to meropenem >16 µg/mL, and synergy on double-disc diffusion with EDTA. Modified carbapenem inactivation method (mCIM) is positive. The most likely resistance mechanism is:

• A Extended-spectrum beta-lactamase (ESBL) production
• B AmpC beta-lactamase hyperproduction
• C KPC (Klebsiella pneumoniae carbapenemase) serine beta-lactamase
• D Metallo-beta-lactamase (MBL) production (NDM, VIM, IMP type) ✓

Explanation

EDTA synergy on double-disc diffusion indicates MBL production (zinc-dependent class B carbapenemases: NDM, VIM, IMP), because EDTA chelates zinc and restores susceptibility. mCIM positivity confirms carbapenemase production. MBL-producing organisms are resistant to all carbapenems and all beta-lactams. KPC is a serine carbapenemase (class A, not inhibited by EDTA); it is detected by mCIM positive + eCIM (EDTA) negative. ESBLs hydrolyze penicillins and cephalosporins but are usually carbapenem-susceptible.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.