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JAC Advance Access published online on February 22, 2005
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dki036
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JAC © The British Society for Antimicrobial Chemotherapy 2005; all rights reserved
Received October 24, 2004
Revised December 20, 2004
Accepted December 29, 2004

Brief report

Disc diffusion-based screening tests for extended-spectrum {beta}-lactamases in Haemophilus influenzae

Stephen G. Tristram 1*, Bülent Bozdogan 2, and Peter C. Appelbaum 2

1 School of Human Life Sciences, University of Tasmania, Launceston, Tasmania 7250, Australia
2 Department of Pathology, Hershey Medical Center, Hershey, PA 17033, USA

* To whom correspondence should be addressed.
Stephen G. Tristram, E-mail: Stephen.Tristram{at}utas.edu.au


  Abstract

Objectives: 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 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 ≥ 5 mm compared with the corresponding plain cephalosporin disc.

Keywords: H. influenzae; ESBLs; TEM.
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