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Journal of Clinical Microbiology, January 2007, p. 199-205, Vol. 45, No. 1
0095-1137/07/$08.00+0     doi:10.1128/JCM.01319-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Effects of Phenotype and Genotype on Methods for Detection of Extended-Spectrum-ß-Lactamase-Producing Clinical Isolates of Escherichia coli and Klebsiella pneumoniae in Norway

Reference Centre for Detection of Antimicrobial Resistance (K-res), Department of Microbiology and Infection Control, University Hospital of North Norway, and Department of Microbiology and Virology, Institute of Medical Biology, Faculty of Medicine, University of Tromsø, Tromsø, Norway,¹ Department of Microbiology, Sørlandet Hospital, Kristiansand, Norway,² Department of Microbiology, Akershus University Hospital, Lørenskog, Norway,³ Department of Medical Microbiology, Cardiff University, Cardiff, United Kingdom,⁴ Norwegian Institute of Public Health, Oslo, Norway⁵

Received 27 June 2006/ Returned for modification 11 August 2006/ Accepted 14 October 2006

Consecutive clinical isolates of Escherichia coli (n = 87) and Klebsiella pneumoniae (n = 25) with reduced susceptibilities to oxyimino-cephalosporins (MICs > 1 mg/liter) from 18 Norwegian laboratories during March through October 2003 were examined for bla_{TEM/SHV/CTX-M} extended-spectrum-ß-lactamase (ESBL) genes, oxyimino-cephalosporin MIC profiles, ESBL phenotypes (determined by the ESBL Etest and the combined disk and double-disk synergy [DDS] methods), and susceptibility to non-ß-lactam antibiotics. Multidrug-resistant CTX-M-15-like (n = 23) and CTX-M-9-like (n = 15) ESBLs dominated among the 50 ESBL-positive E. coli isolates. SHV-5-like (n = 9) and SHV-2-like (n = 4) ESBLs were the most prevalent in 19 ESBL-positive K. pneumoniae isolates. Discrepant ESBL phenotype test results were observed for one major (CTX-M-9) and several minor (TEM-128 and SHV-2/-28) ESBL groups and in SHV-1/-11-hyperproducing isolates. Negative or borderline ESBL results were observed when low-MIC oxyimino-cephalosporin substrates were used to detect clavulanic acid (CLA) synergy. CLA synergy was detected by the ESBL Etest and the DDS method but not by the combined disk method in SHV-1/-11-hyperproducing strains. The DDS method revealed unexplained CLA synergy in combination with aztreonam and cefpirome in three E. coli strains. The relatively high proportion of ESBL-producing E. coli organisms with a low ceftazidime MIC in Norway emphasizes that cefpodoxime alone or both cefotaxime and ceftazidime should be used as substrates for ESBL detection.

Published ahead of print on 1 November 2006.

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