AimsCharacterize the genetic type and resistance mechanisms of 16 carbapenem-resistant Acinetobacter baumannii (CRAB) isolates recovered between January 2010 and March 2011 from US tertiary-care hospital. Methods and ResultsA modified Hodge test demonstrated the presence of carbapenemases, but meropenem and ethylenediaminetetraacetic acid (EDTA) double-disc synergy tests and PCR for metallo--lactamase (MBL) genes werenegative. The genes of ampC -lactamase and efflux pump of adeABC and adeIJK were detected. The presence of oxacillinase (OXA)-like genes, bla(OXA-51-like), bla(OXA-23-like) and bla(OXA-40-like) genes, and insertion sequence ISAba1 in promoter region of bla(OXA-51-like) and bla(OXA-23-like) genes were detected; and confirmed by RT-PCR analyses. The sequencing of bla(OXA-51-like) genes revealed two major alleles, bla(OXA-66-like) (bla(OXA-82)) and bla(OXA-113) from 312 to 688% of isolates respectively. The bla(OXA-23) and bla(OXA-72) genes showed high expression and found co-harbouring bla(OXA-51-like) gene preceded by ISAba-1. All CRAB isolates revealed significant reduction in carO transcription, indicated downregulation of CarO porin system, a potentially independent mechanism of carbapenam resistance. Sequencing of carO gene from representative isolates showed no ISAba1 insertional inactivation. Pulsed-field gel electrophoresis revealed a clonal relationship. ConclusionsCRAB exhibited diversity of mechanisms of carbapenem resistance, and clonal relationship. Significance and Impact of the StudyStudies on distinct outbreaks of CRAB are alarming situation for clinicians.