Journal of Advances in Developmental Research

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Volume 17 Issue 2
2026

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Decoding AmpC β-lactamase production: cefotaxime zone extension as an additional feature

Author(s) Dr. Gopal Agrawal, Dr. Sarfaraz Ahmed, Dr. Nishu Sharma
Country India
Abstract AmpC β-lactamases are an important cause of resistance to cephalosporins in Gram-negative bacilli and are often missed in routine testing. We report a phenotypic observation suggestive of AmpC β-lactamase production noted during routine antimicrobial susceptibility testing by the Kirby–Bauer disk diffusion method. The isolate was resistant to cefoxitin, a recognized screening marker for AmpC production, and showed reduced susceptibility to cefotaxime, while remaining susceptible to imipenem. The absence of zone enhancement around amoxicillin–clavulanate argued against classical extended-spectrum β-lactamase activity. The isolate also appeared to show blunting of inhibition zones between cefotaxime and known inducers, suggesting inducible AmpC activity. An additional, often unnoticed feature was enhancement of the cefotaxime inhibition zone adjacent to the cefoxitin–cloxacillin disk. Careful interpretation of routine disk diffusion plates can help recognize AmpC producers, especially where molecular diagnostics are unavailable. We recommend routine screening for AmpC β-lactamase production in cefoxitin-resistant isolates, with confirmatory inhibitor-based or molecular testing wherever feasible, to guide appropriate antimicrobial therapy and infection control.
Keywords AmpC β-lactamase; Cefoxitin; Cloxacillin; Cefotaxime; Disk diffusion; Antimicrobial susceptibility testing
Field Medical / Pharmacy
Published In Volume 17, Issue 2, July-December 2026
Published On 2026-11-04
DOI https://doi.org/10.71097/IJAIDR.v17.i2.2205
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Crossref DOI prefix of IJAIDR is 10.71097/IJAIDR

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