Multiple drug resistant carbapenemases producing acinetobacter baumannii isolates harbours multiple R-plasmids

Background & objectives: The nosocomial human pathogen Acinetobacter baumannii has high propensity to develop resistance to antimicrobials and to become multidrug resistant (MDR), consequently complicating the treatment. This study was carried out to investigate the presence of resistant plasmids (R-plasmids) among the clinical isolates of A. baumannii. In addition, the study was performed to check the presence of common β-lactamases encoding genes on these plasmids.

Methods: A total of 55 clinical isolates of A. baumannii were included in the study and all were subjected to plasmid DNA isolation, followed by PCR to check the presence of resistance gene determinants such as bla_OXA-23, bla_OXA-51, bla_OXA-58 and bla_IMP-1 on these plasmids that encode for oxacillinase (OXA) and metallo-β-lactamase (MBL) type of carbapenemases. Plasmid curing experiments were carried out on selected isolates using ethidium bromide and acridine orange as curing agents and the antibiotic resistance profiles were evaluated before and after curing.

Results: All the isolates were identified as A. baumannii by 16SrDNA amplification and sequencing. Plasmid DNA isolated from these isolates showed the occurrence of multiple plasmids with size ranging from 500bp to ≥ 25 kb. The percentage of bla_OXA-51 and bla^OXA-23 on plasmids were found to be 78 and 42 per cent, respectively and 20 isolates (36%) carried bla_IMP-1 gene on plasmids. Significant difference was observed in the antibiograms of plasmid cured isolates when compared to their parental ones. The clinical isolates became susceptible to more than two antibiotic classes after curing of plasmids indicating plasmid borne resistance.

Interpretation & conclusions: Our study determined the plasmid mediated resistance mechanisms and occurrence of different resistance genes on various plasmids isolated from MDR A. baumannii. The present findings showed the evidence for antibiotic resistance mediated through multiple plasmids in A. baumannii clinical isolates. This indicates towards a need for preventive measures to avert the dissemination of plasmid resistance determinants in clinical environments.