Plasmid-Mediated Fluoroquinolone Resistance in Pseudomonas aeruginosa and Acinetobacter baumannii

Introduction   Pseudomonas aeruginosa and Acinetobacter baumannii are important pathogens in health care-associated infections. Fluoroquinolone resistance has emerged in these pathogens. In this study, we aimed to determine the occurrence of plasmid-mediated quinolone resistance (PMQR) determinants ( qnrA , qnrB , qnrS , aac(6')-Ib-cr , oqxAB , and qepA ) by polymerase chain reaction (PCR) and the transmissibility of plasmid-borne resistance determinants in clinical isolates of P. aeruginosa and A. baumannii . Materials and Methods  The study included P. aeruginosa (85) and A. baumannii (45) which were nonduplicate, clinically significant, and ciprofloxacin resistant. Antibiotic susceptibility testing was done by disk diffusion method for other antimicrobial agents, namely amikacin, ceftazidime, piperacillin/tazobactam, ofloxacin, levofloxacin, and imipenem. Minimum inhibitory concentration of ciprofloxacin was determined. Efflux pump activity was evaluated using carbonyl-cyanide m-chlorophenylhydrazone (CCCP). The presence of PMQR genes was screened by PCR amplification. Transferability of PMQR genes was determined by conjugation experiment, and plasmid-based replicon typing was performed. Results  Resistance to other classes of antimicrobial agents was as follows: ceftazidime (86.9%), piperacillin/tazobactam (73.8%), imipenem (69.2%), and amikacin (63.8%). The minimal inhibitory concentration (MIC)50 and MIC90 for ciprofloxacin were 64 and greater than or equal to 256 µg/mL, respectively. There was a reduction in MIC for 37 (28.4%) isolates with CCCP. In P. aeruginosa , 12 (14.1%) isolates harbored qnrB , 12 (14.1%) qnrS , 9 (10.5%) both qnrB and qnrS , 66 (77.6%) aac(6')-Ib-cr , and 3 (3.5%) oqxAB gene. In A. baumannii , qnrB was detected in 2 (4.4%), 1 (2.2%) harbored both the qnrA and qnrS , 1 isolate harbored qnrB and qnrS , 21 (46.6%) aac(6')-Ib-cr , and 1 (2.2%) isolate harbored oqxAB gene. Notably, qepA gene was not detected in any of the study isolates. Conjugation experiments revealed that 12 (9.2%) were transferable. Of the transconjugants, seven (58.3%) belonged to IncFII type plasmid replicon, followed by four (33.3%) IncA/C and one (8.3%) IncFIC type. Conclusion  The plasmid-mediated resistance aac(6')-Ib-cr gene is primarily responsible for mediating fluoroquinolone resistance in clinical isolates of P . aeruginosa and A. baumannii . The predominant plasmid type is IncFII.