Overexpression of bla(OXA-58) Gene Driven by ISAba3 Is Associated with Imipenem Resistance in a Clinical Acinetobacter baumannii Isolate from Vietnam

The aim of this study was to investigate genetic structures and expression of bla(OXA-58) gene in five Acinetobacter baumannii clinical isolates recovered from two hospitals in southern Vietnam during 2012-2014. A. baumannii isolates were identified by automated microbiology systems and confirmed by PCR. All isolates were characterized as multidrug resistant by antimicrobial testing using the disk diffusion method. Four imipenem susceptible and one nonsusceptible isolates (MIC > 32 μg·ml(−1)) were identified by E-test. PCR amplification of bla(OXA-58) gene upstream and downstream sequences revealed the presence of ISAba3 at both locations in one multidrug-resistant isolate. Semiquantitation of bla(OXA-51) and bla(OXA-58) gene expression was performed by the 2(-ΔΔCt) method. The bla(OXA-51) gene expression of five isolates showed little difference, but the isolate bearing ISAba3-bla(OXA-58)-ISAba3 exhibited significantly higher bla(OXA-58) mRNA level. Higher β-lactamases activity in periplasmic than cytoplasmic fraction was found in most isolates. The isolate overexpressing bla(OXA-58) gene possessed very high periplasmic enzyme activity. In conclusion, the A. baumannii isolate bearing ISAba3-bla(OXA-58) gene exhibited high resistance to imipenem, corresponding to an overexpression of bla(OXA-58) gene and very high periplasmic β-lactamase activity.