title: Citrobacter koseri and Citrobacter amalonaticus isolates carry highly divergent beta-lactamase genes despite having high levels of biochemical similarity and 16S rRNA sequence homology.
abstract: OBJECTIVE
  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.
authors.affiliation: Bristol Centre for Antimicrobial Research and Evaluation, Department of Pathology & Microbiology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Underwood
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Avison
mesh_terms: D002440:Cefoxitin / Q000494:pharmacology; D002511:Cephalosporins / Q000378:metabolism; D002513:Cephamycins / Q000494:pharmacology; D002954:Citrobacter / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics*; D041041:Citrobacter koseri / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial; D008826:Microbial Sensitivity Tests; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012689:Sequence Homology, Nucleic Acid; D001618:beta-Lactamases / Q000235:genetics*
chemical_list: D002511:Cephalosporins; D002513:Cephamycins; D012336:RNA, Ribosomal, 16S; D002440:Cefoxitin; D001618:beta-Lactamases; C021720:nitrocefin