In vivo selection of reduced susceptibility to carbapenems in Acinetobacter baumannii related to ISAba1-mediated overexpression of the natural bla(OXA-66) oxacillinase gene

Two clonally related Acinetobacter baumannii isolates, A1 and A2, were obtained from the same patient. Isolate A2, selected after an imipenem-containing treatment, showed reduced susceptibility to carbapenems. This resistance pattern was related to insertion of the ISAba1 element upstream of the naturally occurring bla(OXA-66) carbapenemase gene as demonstrated by sequencing, reverse transcription-PCR analysis, and inactivation of the bla(OXA-66) gene.