Four isolates (KP1–4) were recovered sequentially from an infected patient. Whilst KP1–3 were resistant to all β-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 and genes and a 65-kb plasmid that was not transferred by conjugation into , and harboured the plasmid-mediated AmpC β-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 isolate harbouring extended-spectrum β-lactamse and plasmid-mediated cephalosporinase genes following prolonged imipenem exposure.