Yokenella regensburgei is an opportunistic human pathogen that phenotypically resembles Hafnia alvei. The susceptibility of 10 Y. regensburgei strains to 75 antimicrobial agents was examined, applying a microdilution procedure in cation-adjusted Mueller-Hinton broth (CAMHB) and IsoSensitest broth (ISB). beta-Lactamases were characterized phenotypically with beta-lactamase activity and induction assays. Genotypically, PCR experiments applying degenerated primer pairs for the detection of AmpC beta-lactamase genes were performed. Examining the phenotypic properties of Yokenella and 76 H. alvei strains with commercial identification systems and conventional tests, a database for an accurate biochemical separation of Y. regensburgei from H. alvei was established. In CAMHB, all tested yokenellae were resistant or at least of intermediate susceptibility to penicillin G, oxacillin, amoxicillin, amoxicillin-clavulanate, cefaclor, cefazoline, loracarbef, cefoxitin, all tested macrolides, lincosamides, streptogramins, ketolides, fusidic acid, glycopeptides, linezolid, and rifampicin. All Yokenella strains were sensitive to several beta-lactams, all tested aminoglycosides, chloramphenicol, folate-pathway inhibitors, fosfomycin, nitrofurantion, quinolones, and tetracyclines. In ISB, the minimum inhibitory concentration (MIC) values of several beta-lactams were one to four MIC doubling dilution steps lower than those found in CAMHB (depending on the beta-lactam). All yokenellae yielded specific amplification products for ampC, and all of these strains expressed beta-lactamases that were strongly inducible. Hydroxyproline amidase, maltosidase, tri-peptidase, proline deaminase, catalase reaction, Voges-Proskauer test, and fermentation of glycerol, melibiose and myo-inositol were suitable parameters to separate Y. regensburgei from H. alvei.

Elongation factor Tu (EF-Tu), encoded by tuf genes, carries aminoacyl-tRNA to the ribosome during protein synthesis. Duplicated tuf genes (tufA and tufB), which are commonly found in enterobacterial species, usually coevolve via gene conversion and are very similar to one another. However, sequence analysis of tuf genes in our laboratory has revealed highly divergent copies in 72 strains spanning the genus Yersinia (representing 12 Yersinia species). The levels of intragenomic divergence between tufA and tufB sequences ranged from 8.3 to 16.2% for the genus Yersinia, which is significantly greater than the 0.0 to 3.6% divergence observed for other enterobacterial genera. We further explored tuf gene evolution in Yersinia and other Enterobacteriaceae by performing directed sequencing and phylogenetic analyses. Phylogenetic trees constructed using concatenated tufA and tufB sequences revealed a monophyletic genus Yersinia in the family Enterobacteriaceae. Moreover, Yersinia strains form clades within the genus that mostly correlate with their phenotypic and genetic classifications. These genetic analyses revealed an unusual divergence between Yersinia tufA and tufB sequences, a feature unique among sequenced Enterobacteriaceae and indicative of a genus-wide loss of gene conversion. Furthermore, they provided valuable phylogenetic information for possible reclassification and identification of Yersinia species.