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

We sought to determine whether extended-spectrum beta-lactamases (ESBLs) and AmpC beta-lactamases (derepressed and inducible), alone and in combination, could be detected in unidentified members of the Enterobacteriaceae using a simple, overnight disc diffusion test. The 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. AmpC beta-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 mu g) discs. ESBLs, in the presence of AmpC beta-lactamases, could be detected using a difference of > 5 mm between cefepime/clavulanate and cefepime (30 mu g) discs. AmpC beta-lactamases could be detected using a difference of > 14 mm between cefepime/clavulanate and cefpodoxime/clavulanate discs. Inducible AmpC beta-lactamases could be discerned by observing the blunting of the cefpodoxime or cefpodoxime/clavulanate zones in proximity to cefoxitin (30 mu g) discs.