Studies on Acinetobacter baumannii involving multiple mechanisms of carbapenem resistance.

Abstract

Characterize 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. A modified Hodge test demonstrated the presence of carbapenemases, but meropenem and ethylenediaminetetraacetic acid (EDTA) double-disc synergy tests and PCR for metallo-β-lactamase (MBL) genes werenegative. The genes of ampC β-lactamase and efflux pump of adeABC and adeIJK were detected. The presence of oxacillinase (OXA)-like genes, blaOXA-51-like , blaOXA-23-like and blaOXA-40-like genes, and insertion sequence ISAba1 in promoter region of blaOXA-51-like and blaOXA-23-like genes were detected; and confirmed by RT-PCR analyses. The sequencing of blaOXA-51-like genes revealed two major alleles, blaOXA-66-like (blaOXA-82 ) and blaOXA-113 from 31·2 to 68·8% of isolates respectively. The blaOXA-23 and blaOXA-72 genes showed high expression and found co-harbouring blaOXA-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. CRAB exhibited diversity of mechanisms of carbapenem resistance, and clonal relationship. Studies on distinct outbreaks of CRAB are alarming situation for clinicians.