Characterization and nucleotide sequence of a Klebsiella oxytoca cryptic plasmid encoding a CMY-type beta-lactamase: Confirmation that the plasmid-mediated cephamycinase originated from the Citrobacter freundii AmpC beta-lactamase

Plasmid pTKH11, originally obtained by electroporation of a Klebsiella oxyt oca plasmid preparation into Escherichia call XAC, expressed a high level o f an AmpC-like beta-lactamase, The enzyme, designated CMY-5, conferred resi stance to extended-spectrum beta-lactams in E. coli; nevertheless, the phen otype was cryptic in the K. oxytoca donor. Determination of the complete nu cleotide sequence of pTKH11 revealed that the 8,193-bp plasmid encoded seve n open reading frames, including that for the CMY-5 beta-lactamase (bla(CMY -5)). The bla(CMY-5) product was similar to the plasmidic CMY-2 beta-lactam ase of K. pneumoniae and the chromosomal AmpC of Citrobacter freundii, with 99.7 and 97.0% identities, respectively; there was a substitution of pheny lalanine in CMY-5 for isoleucine 105 in CMY-2. bla(CMY-5) was followed by t he Blc and SugE genes of C. freundii, and this cluster exhibited a genetic organization identical to that of the ampC region on the chromosome of C. f reundii; these results confirmed that C. freundii AmpC was the evolutionary origin of the plasmidic cephamycinases, In the K. oxytoca host, the copy n umber of pTKH11 was very low and the plasmid coexisted with plasmid pNBL63, Analysis of the replication regions of the two plasmids revealed 97% seque nce similarity in the RNA I transcripts; this result implied that the two p lasmids might be incompatible. Incompatibility of the two plasmids might ex plain the cryptic phenotype of bla(CMY-5) in K. oxytoca through an exclusio n effect on pTKH11 by resident plasmid pNBL63.