Background : Acinetobacter baumannii is a glucose-nonfermenting gram-negative rod and is a well-recognized nosocomial pathogen. In recent years, A. baumannii strains showing resistance to carbapenems by producing metallo-β-lactamases or OXA-type β-lactamases have increased, and it is considered to be a serious clinical problem. But genotypes of carbapenemases produced by A. baumannii isolates in Korea have been rarely reported. The purpose of this study was to investigate the prevalence of imipenem-resistant A. baumannii and to determine the mechanism of resistance.

Methods : During the period of January through September, 2003, susceptibilities to imipenem of A. baumannii isolates from patients admitted in Kosin University Gospel Hospital in Busan, Korea were investigated. The modified Hodge and EDTA-disk synergy tests were performed for screening of carbapenemase and metallo-β-lactamase-production. Minimal inhibitory concentrations (MICs) were determined by the agar dilution method. For detection of IMP, VIM and OXA-type β-lactamases genes, polymerase chain reactions (PCR) were performed, and the DNA sequences of OXA-type β-lactamases genes were determined by using the dideoxy-chain termination method. The isoelectric points of β-lactamases were determined by isoelectric focusing. Pulsed-field gel electrophresis (PFGE) of the SmaI-digested genomic DNA was performed.

Results : A total of 193 strains of A. baumannii were collected from patients during the surveillance period. Twenty-seven percents (52/193) of A. baumannii isolates were resistant to imipenem. Among the 52 imipenem-resistant isolates, 41 isolates (78.8%) showed positive results in the modified Hodge test, but none of the isolates showed positive results in the EDTA-disk synergy test. Thirty-eight modified Hodge test-positive isolates harbored bla_OXA-23 gene, but none of the isolates harbored IMP- or VIM-type metallo-β-lactamases genes. Analytical isoelectric focusing revealed that all the 38 isolates had a nitrocefin-positive band at pI of 6.65. Thirty-five OXA-23-producing isolates showed a similar PFGE pattern when digested by SmaI endonuclease.

Conclusion : Thirty-eight clinical isolates of A. baumannii acquired resistance to imipenem by producing OXA-23 β-lactamase. Among them were 35 isolates thought to be originated from the same source, because they contained a similar chromosomal type. To the best of our knowledge, this is the first time that OXA-23 β-lactamase has been detected in Korea. (Korean J Clin Microbiol 2004;7(2):139-147)