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We investigated the mechanism of carbapenem resistance in 10Acinetobacter baumannii strains isolated from the United States and Mexico between 2005 and 2009. The detection of known metallo-β-lactamase or carbapenem-hydrolyzing oxacillinase (OXA) genes by PCR was negative. The presence of plasmid-encoded carbapenem resistance genes was investigated by transformation ofA. baumannii ATCC 17978. Shotgun cloning experiments and sequencing were performed, followed by the expression of a novel β-lactamase inA. baumannii . Three novel OXA enzymes were identified, OXA-235 in 8 isolates and the amino acid variants OXA-236 (Glu173-Val) and OXA-237 (Asp208-Gly) in 1 isolate each. The deduced amino acid sequences shared 85% identity with OXA-134, 54% to 57% identities with the acquired OXA-23, OXA-24, OXA-58, and OXA-143, and 56% identity with the intrinsic OXA-51 and, thus, represent a novel subclass of OXA. The expression of OXA-235 inA. baumannii led to reduced carbapenem susceptibility, while cephalosporin MICs were unaffected. Genetic analysis revealed thatbla OXA-235 ,bla OXA-236 , andbla OXA-237 were bracketed between two ISAba1 insertion sequences. In addition, the presence of these acquired β-lactamase genes might result from a transposition-mediated mechanism. This highlights the propensity ofA. baumannii to acquire multiple carbapenem resistance determinants.

OXA-235, a Novel Class D β-Lactamase Involved in Resistance to Carbapenems in Acinetobacter baumannii