ARS | Publication request: Molecular analysis of imipenem-resistant Acinetobacter baumannii isolated from US service members wounded in Iraq, 2003

Submitted to: Epidemiology and Infection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 12, 2011
Publication Date: December 1, 2012
Citation: Huang, X., Chahine, M.A., Frye, J.G., Cash, D.M., Lesho, E.P., Craft, D.W., Lindler, L.E., Nikolich, M.P. 2012. Molecular analysis of imipenem-resistant Acinetobacter baumannii isolated from US service members wounded in Iraq, 2003–2008. Epidemiology and Infection. 140(12):2302-2307.

Interpretive Summary: Acinetobacter baumannii-calcoaceticus complex (ABC) is a bacterium that causes wound infections in many combat casualties. Development of mutli-drug resistance (MDR; defined as resistance to = 2 antimicrobials) complicates treatment options. This is of particular concern when MDR includes resistance to imipenem which is one of the last drugs available to treat MDR ABC infections. An increase in ABC strains which are MDR has complicated treatment of these wounds in U.S. personnel participating in Operation Iraqi Freedom. In this study, DNA from 298 ABC isolates stored in the Walter Reid Army Institute of Research strain collection were characterized to determine how closely they matched on a genetic level. Pulse field gel electrophoresis (PFGE) was used to classify them into 67 distinct PFGE types (PFTs). Microarray analysis of DNA from isolates representative of the major PFTs detected the presence of several antimicrobial resistance genes. Only isolates that hybridized with a probe for blaOXA-23/27 were resistant to imipenem (IR). DNA Southern blots demonstrated that the blaOXA-23 was plasmid-borne or both plasmid and chromosomally located. A plasmid carrying blaOXA-23 transformed into a susceptible ABC strain conferred IR to the recipient. The distribution of the IR-ABC with specific PFTs implied nosocomial spread in military treatment facilities.

Technical Abstract: Clonal spread and global dissemination of imipenem resistant (IR) A. baumannii-A. calcoaceticus complex (ABC) have been reported in recent years. However, the epidemiological features of the IR-ABCs in military treatment facilities (MTFs) have not been systematically studied. In this study, 298 ABC strains from a historical collection isolated from U.S. service members injured in Iraq and hospitalized in different domestic and overseas MTFs from 2003 through 2008 were characterized genotypically by PFGE, plasmid profile and phenotypically by antimicrobial susceptibility tests (AST). DNA microarray, PCR, Southern blot and plasmid transfer were used to characterize imipenem resistance in this study. PFGE revealed 67 PFGE types. AST detected 46 IR-ABC isolates. PCR results showed that 90% of the IR-ABC isolates carried blaOXA-23 and 2% carried blaOXA-58. IR-ABCs tended to cluster by PFGE in association with isolation date and location. Microarray analysis indicated only those that hybridized with a probe for blaOXA-23/27 were resistant to imipenem. Southern blots demonstrated that the blaOXA-23 was either plasmid-borne or both plasmid and chromosomally located. A plasmid carrying blaOXA-23 transformed into a susceptible ABC strain conferred imipenem resistance to the recipient. Genetic characterization combined with AST provided accurate information on imipenem resistance in the ABC strains studied. The clonal distribution of the IR-ABC implied nosocomial dissemination in the MTFs. The blaOXA-23 appeared to be responsible for imipenem resistance for the majority of ABCs studied. The blaOXA-23 was carried on plasmids or on the chromosomal with an ISAbaI upstream of the gene, suggesting the possibility of horizontal transfer of this resistance gene.