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J Antimicrob Chemother. 2004 Jun;53(6):1076-80. Epub 2004 May 5.

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

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



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.


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.


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.


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.

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