Detection of carbapenemase genes in acinetobacter baumannii

Carbapenems are among the drugs of choice for treatment of multidrug-resistant acinetobacter baumannii infections. However, emergence of carbapenem resistance in A. baumannii has been reported worldwide. The present study identified carbapenemase genes in A. baumannii and investigated the prevalence of carbapenem resistance of the 501 A. baumannii isolated from patients at the King Chulalongkorn Memorial Hospital, Bangkok during January 2004 to August 2007. The minimal inhibitory concentrations (MICs) of imipenem and meropenem were determined by agar dilution. Resistance rates were 82.4% and 81.8% for imipenem and meropenem, respectively. All carbapenem-resistant isolates had carbapenemase activity by modified Hodge test. Metallo- β-lactamases were not detected in any isolate when screened by imipenem-EDTA double-disk synergy test. Screening for carbapenemase genes in the 501 A. baumannii by PCR revealed the presence of blaOXA-23-like, blaOXA-24-like, blaOXA-51-like, and blaOXA-58-like, and the absence of blaIMP-like, and blaVIM-like. All 501 A. baumannii isolates carried the blaOXA-51-like gene. The isolates carrying only blaOXA-51-like were susceptible to carbapenems whereas the isolates carrying blaOXA-51-like together with other blaOXA-like, including blaOXA-23-like, blaOXA-24-like and blaOXA-58-like were resistant to carbapenems. The presence of blaOXA-51-like together with blaOXA-23-like were detected in the majority of carbapenem-resistant A. baumannii isolates (93.2%), suggesting that blaOXA-23-like, plays a major role in carbapenem resistance. DNA sequences of the entire blaOXA-23-like, blaOXA-24-like, blaOXA-51-like and blaOXA-58-like each from 10 representative isolates, were analyzed. The results revealed that the blaOXA-23-like, blaOXA-24-like and blaOXA-58-like, from their 10 representative were blaOXA-23, blaOXA-72 and blaOXA-58 respectively, whereas those of the blaOXA-51-like were blaOXA-68 (5 isolates), blaOXA-66 (2 isolates) and blaOXA-64, blaOXA-67 and a novel blaOXA-51-like (1 isolate each). Nucleotide and amino acid sequences of this novel enzyme showed 99% identity to blaOXA-51 and OXA-51