A simple disc diffusion method for detecting AmpC and extended-spectrum β-lactamases in clinical isolates of Enterobacteriaceae


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Abstract

BackgroundWe sought to determine whether extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases (derepressed and inducible), alone and in combination, could be detected in unidentified members of the Enterobacteriaceae using a simple, overnight disc diffusion test.MethodsThe genetic basis of antibiotic resistance in cephalosporin-resistant wild-type (n=140) and culture collection (n=140) isolates of Enterobacteriaceae was determined using PCR. A scheme for detecting these resistance mechanisms phenotypically was devised using five antibiotic discs: cefpodoxime ± clavulanate; cefepime ± clavulanate and cefoxitin.Results and conclusionsAmpC β-lactamases (derepressed and inducible) and ESBLs, alone and in combination, could reliably be detected using a disc diffusion method. ESBLs alone could be detected on the basis of a difference of >5 mm between cefpodoxime/clavulanate and cefpodoxime (10 µg) discs. ESBLs, in the presence of AmpC β-lactamases, could be detected using a difference of >5 mm between cefepime/clavulanate and cefepime (30 µg) discs. AmpC β-lactamases could be detected using a difference of >14 mm between cefepime/clavulanate and cefpodoxime/clavulanate discs. Inducible AmpC β-lactamases could be discerned by observing the blunting of the cefpodoxime or cefpodoxime/clavulanate zones in proximity to cefoxitin (30 µg) discs.

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