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

We investigated the mechanism of carbapenem resistance in 10 Acinetobacter baumannii strains isolated from the United States and Mexico between 2005 and 2009. The detection of known metallo-beta-lactamase or carbapenem-hydrolyzing oxacillinase (OXA) genes by PCR was negative. The presence of plasmid-encoded carbapenem resistance genes was investigated by transformation of A. baumannii ATCC 17978. Shotgun cloning experiments and sequencing were performed, followed by the expression of a novel beta-lactamase in A. 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 in A. baumannii led to reduced carbapenem susceptibility, while cephalosporin MICs were unaffected. Genetic analysis revealed that bla(OXA-235), bla(OXA-236), and bla(OXA-237) were bracketed between two ISAba1 insertion sequences. In addition, the presence of these acquired beta-lactamase genes might result from a transposition-mediated mechanism. This highlights the propensity of A. baumannii to acquire multiple carbapenem resistance determinants.