A 62-year-old diabetic patient develops a hospital-acquired pneumonia on day 8 of ICU admission. Bronchoalveolar lavage grows Klebsiella pneumoniae. Disk diffusion shows resistance to all third-generation cephalosporins, but the isolate is initially reported sensitive to carbapenems. The combined disk test with clavulanate shows ≥5 mm increase in zone diameter. What resistance mechanism does this isolate carry?

• A KPC (Klebsiella pneumoniae carbapenemase)
• B AmpC beta-lactamase
• C OXA-48 carbapenemase
• D Extended-spectrum beta-lactamase (ESBL) ✓

Explanation

The combined disk test (cephalosporin ± clavulanate) is the confirmatory test for ESBL production; a ≥5 mm increase in zone diameter when clavulanate is added confirms ESBL. ESBL-producing organisms typically resist third-generation cephalosporins but remain sensitive to carbapenems, which distinguishes them from carbapenemase producers such as KPC and OXA-48. AmpC beta-lactamases are not inhibited by clavulanate and show no enhancement zone.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.