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JAC Advance Access published online on May 5, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh235
© 2004 by The British Society for Antimicrobial Chemotherapy
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Received February 25, 2004
Accepted March 12, 2004

Brief report

Citrobacter koseri and Citrobacter amalonaticus isolates carry highly divergent {beta}-lactamase genes despite having high levels of biochemical similarity and 16S rRNA sequence homology

Sarah Underwood 1 Matthew B. Avison 1*

1 Bristol Centre for Antimicrobial Research and Evaluation, Department of Pathology & Microbiology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK

* To whom correspondence should be addressed. E-mail: Matthewb.Avison{at}bris.ac.uk.


  Abstract

Objectives: Isolates previously identified as Citrobacter diversus are now known as Citrobacter koseri. We measured sequence variation at the {beta}-lactamase structural gene among a group of clinical isolates originally identified as C. diversus by API 20E profiling.

Methods: {beta}-Lactamase and 16S rRNA genes were amplified by PCR and sequenced by standard methods. {beta}-Lactamase induction was attempted in liquid-grown cultures using cefoxitin. Nitrocefin hydrolysis assays were performed using a spectrophotometer.

Results: Analysis of 16S rRNA gene sequences showed that Citrobacter spp. isolates with an inducible {beta}-lactamase gene, cdiA, closely related to ‘C. koseri ’ NF85 and ULA27 are actually Citrobacter amalonaticus. C. koseri isolates, whose identities were confirmed by 16S rRNA sequencing, produce a class A {beta}-lactamase, Cko, constitutively at low levels. The cko and cdiA {beta}-lactamase genes share <45% identity.

Conclusions: We have confirmed that cko is a {beta}-lactamase gene carried by C. koseri, and that isolates previously identified as ‘C. koseri ’, but carrying the cdiA {beta}-lactamase gene are C. amalonaticus. Thus, {beta}-lactamase-gene-specific PCR may provide a valuable tool to differentiate these biochemically homogeneous Citrobacter species.

Key Words: Keywords: Citrobacter, {beta}-lactamases, phylogenetics, induction


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