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

Helen Derbyshire, Gemma Kay, Katie Evans, Carmel Vaughan, Umadevi Kavuri, Trevor Winstanley

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16 Citations (Scopus)


BACKGROUND: 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.

METHODS: 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.

RESULTS AND CONCLUSIONS: 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 microg) discs. ESBLs, in the presence of AmpC beta-lactamases, could be detected using a difference of >5 mm between cefepime/clavulanate and cefepime (30 microg) 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 microg) discs.

Original languageEnglish
Pages (from-to)497-501
Number of pages5
JournalJournal of Antimicrobial Chemotherapy
Issue number3
Publication statusPublished - Mar 2009


  • Anti-Bacterial Agents
  • Bacteriological Techniques
  • DNA, Bacterial
  • Enterobacteriaceae
  • Enterobacteriaceae Infections
  • Humans
  • Microbial Sensitivity Tests
  • Polymerase Chain Reaction
  • beta-Lactam Resistance
  • beta-Lactamases
  • beta-Lactams

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