We sought to determine whether extended-spectrum b-lactamases (ESBLs) and AmpC blactamases (derepressed and inducible), alone and in combination, could be detected in unidentified members of the Enterobacteriaceae using a simple, overnight disc diffusion test. Methods: The genetic basis of antibiotic resistance in cephalosporin-resistant wild-type (n5140) and culture collection (n5 140) isolates of Enterobacteriaceae was determined using PCR. A scheme for detecting these resistance mechanisms

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... enotypically was devised using five antibiotic discs: cefpodoxime+ + + + + clavulanate; cefepime + + + + +clavulanate and cefoxitin. Results and conclusions: AmpC b-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 mg) discs. ESBLs, in the presence of AmpC b-lactamases, could be detected using a difference of >5 mm between cefepime/clavulanate and cefepime (30 mg) discs. AmpC b-lactamases could be detected using a difference of >14 mm between cefepime/clavulanate and cefpodoxime/clavulanate discs. Inducible AmpC b-lactamases could be discerned by observing the blunting of the cefpodoxime or cefpodoxime/clavulanate zones in proximity to cefoxitin (30 mg) discs.