In vivo selection of imipenem-resistant Klebsiella pneumoniae producing extended-spectrum beta-lactamase CTX-M-15 and plasmid-encoded DHA-1 cephalosporinase.

Four Klebsiella pneumoniae isolates (KP1-4) were recovered sequentially from an infected patient. Whilst KP1-3 were resistant to all beta-lactams except carbapenems, KP4 recovered after 24 days of imipenem-containing treatment showed additional resistance to carbapenems. No carbapenem hydrolysis could be identified in KP4. Molecular characterisation revealed that KP1-4 were indistinguishable by pulsed-field gel electrophoresis, contained a 95-kb self-transferable plasmid harbouring bla(CTXM-15) and bla(TEM-1) genes and a 65-kb plasmid that was not transferred by conjugation into Escherichia coli, and harboured the plasmid-mediated bla(DHA-1) AmpC beta-lactamase gene. In addition, KP4 failed to express OmpK36 owing to a point mutation leading to a premature stop of the protein. This study demonstrates development of carbapenem resistance related to loss of OmpK36 expression in a K. pneumoniae isolate harbouring extended-spectrum beta-lactamase and plasmid-mediated cephalosporinase genes following prolonged imipenem exposure.