Disc diffusion-based screening tests for extended-spectrum β-lactamases in Haemophilus influenzae

Objectives: To establish a suitable screening test for the detection of extended-spectrum β-lactamases (ESBLs) in Haemophilus influenzae. Methods: Double disc diffusion tests using cephalosporin discs adjacent to an amoxicillin/clavulanate 3 μg disc, prediffusion tests, where an amoxicillin/clavulanate 3 μg disc was prediffused before being replaced with a cephalosporin disc, and combination cephalosporin/clavulanate discs were used to screen recombinant H. influenzae expressing cloned ESBLs. Methods initially determined to be suitable were validated by testing a range of clinical strains. Results: Enhanced zones around cephalosporin discs due to synergy with an adjacent amoxicillin/clavulanate disc were not reliably demonstrated with the recombinant H. influenzae strains expressing ESBLs. Cefotaxime 5 μg or 30 μg and cefpodoxime 10 μg discs prediffused with an amoxicillin/clavulanate 3 μg disc, and cefotaxime 30 μg/clavulanate 10 μg and cefpodoxime 10 μg/clavulanate 1 μg combination discs all gave zone sizes ≥5 mm greater than the corresponding plain disc for all recombinant ESBL-producing strains. None of the clinical strains gave enhanced zones. Conclusions: Cefotaxime 5 or 30 μg or cefpodoxime 10 μg discs prediffused with an amoxicillin/clavulanate 3 μg disc, or cefotaxime 30 μg/clavulanate 10 μg and cefpodoxime 10 μg/clavulanate 1 μg combination discs are all suitable for screening H. influenzae isolates for the presence of ESBLs. ESBL-producing strains should show an increase in zone diameter of ≥5mm compared with the corresponding plain cephalosporin disc. © The Author 2005. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.