Increase of multidrug efflux pump expression in fluoroquinoloneresistant Salmonella mutants induced by ciprofloxacin selective pressure

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 ciprofloxacinresistant 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 pumprelated 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.