Characteristics of 32 Acinetobacter baumanni isolates co-harboring OXA-Carbapenemases and 16S rRNA methylase gene

Multiple drug-resistant strains of Acinetobacter have become common in hospitals worldwide. The problem becomes more acute with existence of hidden Carbapanemase-resistance genes that co-exist with other resistance determinants. Based on this, this research aims to determine the occurrence and co-existence of OXA-type carbapenemases and 16S rRNA methylase gene among clinical isolates of A. baumannii in a Jilin hospital, China. Between January 2012 to December 2013, 32 A. baumanii non-repetitive strains were isolated from nosocomial clinical specimens. Susceptibility to antibiotics was determined by the standard microdilution method and phenotypic testing was used to detect the presence of carbapenemases. PCR was performed using specific primers for detection of class D carbapenemase genes and 16S rRNA methylase gene. Results revealed that all imipinem resistant A. baumannii isolates were resistant to all antibiotics tested except polymycin E. The isolates harbored different Oxacillin Type Carbapenemases, that is, 62.5% of the isolates harbored OXA-51, 37.5% OXA-23, 18.8% OXA-58, 3.1% OXA-24 and 59.4% harbored 16S rRNA methylase gene. Of the 19 isolates harboring 16S rRNA methylase gene, 47.4% coexisted with at least one oxacillinase gene while 40.6% of the 32 A. baumanii isolates harbored more than one different oxacillinase gene. At least one oxacillinase gene was detected in 28.1% of the isolates that were phenotypically negative for carbapenemase production. The current study therefore shows co-existence of resistance determinant genes and existence of hidden genes responsible for resistance. Multiple mechanisms are likely to work in synergism to produce this phenotype.