title: Disc diffusion-based screening tests for extended-spectrum beta-lactamases in Haemophilus influenzae.
abstract: OBJECTIVE
  To establish a suitable screening test for the detection of extended-spectrum beta-lactamases (ESBLs) in Haemophilus influenzae.
  
  
  METHODS
  Double disc diffusion tests using cephalosporin discs adjacent to an amoxicillin/clavulanate 3 microg disc, prediffusion tests, where an amoxicillin/clavulanate 3 microg 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 microg or 30 microg and cefpodoxime 10 microg discs prediffused with an amoxicillin/clavulanate 3 microg disc, and cefotaxime 30 microg/clavulanate 10 microg and cefpodoxime 10 microg/clavulanate 1 microg combination discs all gave zone sizes > or = 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 microg or cefpodoxime 10 microg discs prediffused with an amoxicillin/clavulanate 3 microg disc, or cefotaxime 30 microg/clavulanate 10 microg and cefpodoxime 10 microg/clavulanate 1 mug 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 > or = 5 mm compared with the corresponding plain cephalosporin disc.
authors.affiliation: School of Human Life Sciences, University of Tasmania, Launceston, Tasmania 7250, Australia. Stephen.Tristram@utas.edu.au
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Tristram
authors.forename: Bülent
authors.initials: B
authors.lastname: Bozdogan
authors.forename: Peter C
authors.initials: PC
authors.lastname: Appelbaum
mesh_terms: D000658:Amoxicillin / Q000494:pharmacology; D019980:Amoxicillin-Potassium Clavulanate Combination / Q000494:pharmacology; D002439:Cefotaxime / Q000494:pharmacology; D015296:Ceftizoxime / Q000031:analogs & derivatives* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006193:Haemophilus influenzae / Q000187:drug effects* / Q000201:enzymology*; D008826:Microbial Sensitivity Tests / Q000379:methods*; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000378:metabolism*; D000097623:Cefpodoxime
chemical_list: D000658:Amoxicillin; D019980:Amoxicillin-Potassium Clavulanate Combination; D002439:Cefotaxime; D015296:Ceftizoxime; D001618:beta-Lactamases; D000097623:Cefpodoxime