Characterization of Ciprofloxacin Resistance in Laboratory-Derived Mutants of Vibrio parahaemolyticus with qnr Gene.

Ciprofloxacin, a broad-spectrum fluoroquinolone, is a bactericidal antibiotic targeting DNA gyrase and DNA topoisomerase IV encoded by the gyrA and parC genes. Resistance to fluoroquinolones requires the accumulation of multiple mutations including those that alter target genes and increase drug efflux. To examine the development of fluoroquinolones resistance in Vibrio parahaemolyticus, ciprofloxacin induction and selection was used to obtain several resistant V. parahaemolyticus mutants, which showed decreased susceptibilities to quinolones, and increased or decreased susceptibility to other structurally unrelated antibiotics. Quinolone resistance-determining region mutations were characterized, and it was found that gyrA mutations occurred in some of the high-level resistant mutants although qnr was present in both wild-type susceptible and resistant mutant strains. The mutants showed increased qnr expression and exposure to sub-inhibitory concentrations of ciprofloxacin caused a further increase in qnr expression independently of the SOS system. Two mutants demonstrated increased expression of the VmeCD-VpoC pump gene that promotes quinolone efflux. In addition, some of the high-level resistance mutants significantly decreased bacterial fitness. These data suggested that multiple genes contributed to the enhanced ciprofloxacin resistance appeared in V. parahaemolyticus and that acquisition of ciprofloxacin resistance impaired bacterial fitness.