A burn wound isolate of Pseudomonas aeruginosa is tested by the E-test method. The MIC for meropenem reads 8 µg/mL (EUCAST breakpoint: sensitive ≤2, resistant ≥8). The modified carbapenem inactivation method (mCIM) test is positive (turbid broth). The Carba NP test is also positive and turns yellow rapidly. Which carbapenemase class is MOST likely present?

• A Class A serine carbapenemase (KPC)
• B Class B metallo-beta-lactamase (MBL) ✓
• C Class D OXA-type carbapenemase
• D AmpC overexpression with porin loss

Explanation

VIM and NDM are metallo-beta-lactamases (Class B) frequently found in Pseudomonas. The Carba NP test detects carbapenemase by hydrolysis of imipenem turning the indicator yellow; rapid colour change in Pseudomonas most commonly indicates MBL (VIM type) in Indian ICUs. However, distinguishing MBL from serine carbapenemase requires EDTA inhibition: if EDTA addition increases the imipenem zone in disk diffusion, it confirms MBL. AmpC + porin loss does not yield a positive Carba NP test.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.