A Klebsiella pneumoniae isolate from a urine culture is resistant to all cephalosporins but susceptible to piperacillin-tazobactam and carbapenems. Phenotypic confirmation testing using the combined disc method with clavulanic acid shows a ≥5 mm increase in zone diameter. Which resistance mechanism is most likely?
- A AmpC beta-lactamase production
- B Carbapenemase (KPC) production
- C Extended-spectrum beta-lactamase (ESBL) production ✓
- D Porin channel loss (OmpK35/OmpK36 deletion)
Correct answer: C. Extended-spectrum beta-lactamase (ESBL) production
Explanation
ESBL-producing organisms show resistance to third-generation cephalosporins but are inhibited by beta-lactamase inhibitors such as clavulanic acid, which explains the ≥5 mm zone increase with the combined disc test — the hallmark of ESBL phenotypic confirmation. AmpC beta-lactamases are not inhibited by clavulanic acid and show resistance even to cephamycins. KPC-producing organisms are resistant to carbapenems as well. Porin loss alone does not produce synergy with clavulanic acid in disc testing.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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