A urine culture isolate of E. coli shows resistance to ceftriaxone, cefotaxime, and ceftazidime but is susceptible to cefoxitin and imipenem. The double-disc synergy test (DDST) using cefotaxime and cefotaxime+clavulanate discs shows a ≥ 5 mm increase in zone size with the combination. This confirms:
- A Extended-spectrum beta-lactamase (ESBL) production ✓
- B AmpC beta-lactamase production
- C Carbapenemase (KPC) production
- D OmpK35/36 porin loss
Explanation
The double-disc synergy test detects ESBL production: clavulanate inhibits ESBL enzymes (serine-active-site beta-lactamases), so a ≥ 5 mm increase in inhibition zone with the combination disc vs. alone confirms ESBL. ESBLs hydrolyze oxyimino-cephalosporins (ceftriaxone, cefotaxime, ceftazidime) but not cephamycins (cefoxitin) or carbapenems — matching this antibiogram. AmpC beta-lactamases also hydrolyze cefoxitin; they are NOT inhibited by clavulanate (DDST negative). Carbapenemase production would show imipenem resistance. Porin loss alone would show imipenem resistance without ESBL phenotype.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.