In clinical isolates of Klebsiella oxytoca resistance to cefuroxime and aztreonam was mediated by a β-lactamase, designated KH, (pI 5·25) which could be transferred into Escherichia coli by electroporation, but not by conjugation. The transformants produced two enzymes with pIs 5·25 and 8·4 respectively, and showed resistance to cefuroxinie, aztreonam, cefotaximie and ceftazidime. Substrate and inhibition profiles indicated that KH β-lactamase was different from TEM- or SHV-like enzymes, but similar to chromosomal K1 β-lactamase. The enzyme profile with pI 8·4 was similar to the enzyme from the recipient and showed elevated activity in transformants. The plasmid profiles of the transformants were different from those of their donors. However, a plasmid fragment of the K. oxytoca isolate KH11 hybridized with a plasmid ranging in size from 4·8 to 7·8 kilobases in all the transformants and most of the donors. Gene probes encoding TEM-1 or SHV-1 did not hybridize with plasmid DNA from the K. oxytoca isolates. Furthermore, a probe of the ampC gene did not hybridize with the plasmid but to DNA fragments of the same size in whole cell DNA preparations from the E. coli XAC recipient and the TKH11 transformants. This indicates that no gross rearrangements in the chromosomal β-lactamaze gene region had occurred in the transformants which could explain the increased expression of the pI 8·4 β-lactamase. © 1992 The British Society for Antimicrobial Chemotherapy.
