Fluoroquinolone resistance of Serratia marcescens: involvement of a proton gradient-dependent efflux pump.

To determine the presence of a proton gradient-dependent efflux of fluoroquinolone drugs in Serratia marcescens. Thirteen clinical isolates of S. marcescens were screened for resistance to four fluoroquinolones: ofloxacin, ciprofloxacin, norfloxacin and nalidixic acid by determining MICs. The presence of a proton gradient-dependent efflux mechanism was assessed using ethidium bromide accumulation assays. Drug accumulation studies for norfloxacin, ciprofloxacin and ofloxacin were performed to determine the drug specificity of efflux. Western transfer of cellular proteins, followed by immunodetection using anti-AcrA (Escherichia coli) antibodies were used to demonstrate the presence of a resistance-nodulation-cell division (RND) pump protein. PCR was used to identify a RND pump-encoding gene using primers for two conserved motifs within inner membrane components of RND proteins. A mutant strain of S. marcescens, UOC-67WL, was isolated by culturing the wild-type strain in the presence of ciprofloxacin in T-soy media and was subjected to the same studies as described above for the clinical isolates. Ethidium bromide accumulation assays confirmed the presence of a proton gradient-dependent efflux mechanism in S. marcescens. One clinical isolate, T-861, and the mutant strain, UOC-67WL, were found to efflux ciprofloxacin and ofloxacin. Western immunoblot results confirmed overexpression of an AcrA-like protein in T-861 and UOC-67WL. Sequencing of the PCR product showed the presence of a mexF-like gene, which is overexpressed in nfxC mutants of Pseudomonas aeruginosa. This study reports the presence of a proton gradient-dependent efflux mechanism in S. marcescens.