An E. coli isolate from a UTI shows resistance to cefotaxime and ceftazidime but susceptibility to cefoxitin and carbapenems. The ESBL phenotype is confirmed by the combination disc test showing ≥5 mm increase in inhibition zone with cefotaxime + clavulanic acid versus cefotaxime alone. Which enzyme class is most likely responsible?
- A NDM-1 (New Delhi Metallo-beta-lactamase, class B)
- B CTX-M type ESBL (plasmid-mediated class A beta-lactamase) ✓
- C OXA-48 (class D carbapenemase)
- D AmpC beta-lactamase (class C, cephalosporinase)
Explanation
CTX-M ESBLs are the predominant ESBL type globally (>90% of ESBLs) and in India; they are class A serine beta-lactamases encoded on conjugative plasmids. They preferentially hydrolyse cefotaxime (hence CTX) over ceftazidime (unlike TEM/SHV ESBLs). ESBL confirmation: ≥5 mm zone enhancement with clavulanate (clavulanate inhibits class A ESBLs). Susceptibility to cefoxitin rules out AmpC (AmpC ESBLs are resistant to cefoxitin). Carbapenems remain active because ESBLs are inhibited by clavulanate and do not hydrolyse carbapenems. NDM-1 and OXA-48 are carbapenemases — they cause carbapenem resistance.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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