Mechanisms of carbapenem resistance in acinetobacter baumannii clinical isolates

Acinetobacter baumannii is an important cause of nosocomial infection. Carbapenems are the first line drug for treatment of multidrug-resistant A. baumannii infections. Currently, carbapenem-resistant A. baumannii isolates have been increasingly reported worldwide. In this study, the prevalence and the mechanisms of carbapenem resistance in A. baumannii including the production of carbapenemases, the presence of efflux pump and the loss or decrease of outer membrane protein were investigated. A total of 453 A. baumannii isolated from patients at King Chulalongkorn Memorial Hospital between 2008-2010 were studied. The prevalence of imipenem and meropenem resistance were both 88.7%. Carbapenemase activity was found in all carbapenem-resistant A. baumannii isolates but metallo-β-lactamase enzymes were not detected in any isolates. Screening for the presence of carbapenemase genes by multiplex PCR revealed that blaOXA-51-like was detected in all isolates. Of the 402 carbapenem-resistant A. baumannii isolates, 99.8% carried blaOXA-51-like with blaOXA-23-like, 0.2% haboured blaOXA-51-like with blaOXA-24-like and 4% had blaOXA-51-like with blaOXA-23-like and blaOXA-58-like. The metallo-β-lactamase genes were not found in any isolate. The ISAba1 was found in the upstream region of blaOXA-23 in all 38 representative carbapenem-resistant A. baumannii isolates and ISAba3 was found upstream region of blaOXA-58 in all 4 representative isolates. Sequencing analysis of entire blaOXA-like showed that all 15 representative blaOXA-23-like genes were blaOXA-23 and all 4 blaOXA-58-like genes were blaOXA-58. Twelve representative blaOXA-51-like genes were blaOXA-65 (4 isolates), blaOXA-66 (7 isolates) and blaOXA-69 (1 isolate). Five representative ISAba1 upstream region blaOXA-23-like and all 4 ISAba3 upstream region blaOXA-58-like genes had -35 and -10 sequences. The presence of efflux pump mechanism by using carbonyl cyanide m-chlorophenylhydrazone (CCCP), the efflux pump inhibitor, was determined in all 453 isolates. No carbapenem-resistant A. baumannii isolates had this mechanism. Twenty-three A. baumannii isolates were determined for the loss or decrease of outer membrane protein. Five carbapenem-resistant isolates had decreased 43 kDa outer membrane protein. This study showed high rate of carbapenem-resistant A. baumannii isolates in Thailand and demonstrated that carbapenem resistance was attributed to the production of carbapenemases and the reduction of outer membrane protein. The main mechanism of resistance was the production of OXA-23-like carbapenemases.