Background: In the present study, the resistance mechanisms against carbapenems and aminoglycosides for 23 strains of multi-drug-resistant Acinetobacter baumannii isolated at a university hospital were investigated. 

Methods: The minimal inhibitory concentrations (MICs) were determined via broth microdilution or Etest. The genes encoding OXA-type carbapenemases and 16S rRNA methylase were identified using multiplex PCR, and the amplified products were sequenced. Conjugation experiments were conducted, and an epidemiologic study was performed using enterobacterial repetitive intergenic consensus (ERIC)-PCR. 

Results: In the isolates, the MICs of the tested aminoglycosides, including arbekacin, were ＞1024 μg/ mL; the MICs of aztreonam, cefepime, ceftazidime, and ciprofloxacin ranged from 64 to 128 μg/mL; and the MICs of carbapenem ranged from 32 to 64 μg/ mL, as determined through the broth microdilution test. According to the E-test, the MICs of ampicillin/ sulbactam and colistin were 8 and 0.25 to 0.38 μg/ mL, respectively. Sequence analysis confirmed that all of the isolates expressed carbapenemases OXA- 23 and OXA-66, as well as armA 16S rRNA methylase. In addition, ISAba1 was identified upstream of the gene encoding OXA-23. OXA-23 and armA were not transferred to Escherichia coli J53 cells in the transconjugation experiments. ERIC-PCR molecular fingerprinting produced a single pattern in all cases. 

Conclusion: The co-production of OXA-23 and armA 16S rRNA methylase may be attributed to the multi- drug resistance of the A. baumannii isolates in the present study. Stricter surveillance and more rapid detection are necessary to prevent the spread of this type of resistance in the future. (Korean J Clin Microbiol 2011;14:67-73)