Increase of multidrug efflux pump expression in fluoroquinolone-resistant Salmonella mutants induced by ciprofloxacin selective pressure

• A ciprofloxacin-susceptible Salmonella Istanbul was isolated from chicken meat.
• Ciprofloxacin-resistant mutants were induced from susceptible parental strain.
• Expression levels of efflux pump-related genes were determined using qRT-PCR.
• The mutants had elevated MICs with increased expression of marA, ramA, and acrB.
• The continued use of ciprofloxacin may induce resistant Salmonella mutants.

Multidrug-resistant foodborne pathogens are a leading public health concern, as antimicrobial resistance can lead to therapeutic failure. In this study, a ciprofloxacin-susceptible Salmonella Istanbul (Sal10-FC-KU12) was isolated from chicken meat obtained from a market in Korea to induce ciprofloxacin-resistant mutants (SalML, SalMM, and SalMH). Minimum inhibitory concentrations (MICs) of 12 antibiotics were measured in the presence or absence of an efflux pump inhibitor. Expression levels of efflux pump-related genes (acrB, acrF, marA, ramA, rob, and soxS) were determined using quantitative reverse transcription polymerase chain reaction (qRT-PCR). Elevated MICs for the derived mutants were shown to result from the action of the efflux pump, with increased expression of marA, ramA, and acrB compared with the wild-type strain. The results of this study suggest that continued use of ciprofloxacin might induce the emergence of Salmonella mutants resistant not only to fluoroquinolones, but also to several other classes of antimicrobials.