Characterization of Clinically Isolated Antibiotic-Resistant Salmonella Typhimurium Exposed to Subinhibitory Concentrations of Ceftriaxone and Ciprofloxacin.

Abstract

This study was designed mainly to assess the phenotypic properties of clinically isolated Salmonella Typhimurium exposed to ceftriaxone and ciprofloxacin. The antibiotic susceptibility, β-lactamase activity, efflux activity, bacterial motility, biofilm-forming ability, and gene expression were determined in S. Typhimurium ATCC 19585 and S. Typhimurium CCARM 8009 when exposed to subinhibitory concentrations of ceftriaxone and ciprofloxacin. S. Typhimurium CCARM 8009 was highly resistant to ampicillin, kanamycin, penicillin G, and streptomycin, showing minimum inhibitory concentration values of more than 512 μg/ml, while S. Typhimurium ATCC 19585 showed resistance to erythromycin alone (64 μg/ml). The highest β-lactamase activity was observed in S. Typhimurium CCARM 8009 when exposed to ceftriaxone (8.2 μmol/min/ml), while the least β-lactamase activity was observed in S. Typhimurium ATCC 19585. Compared to S. Typhimurium CCARM 8009, the ethidium bromide (EtBr) accumulation was considerably increased in S. Typhimurium ATCC 19585 when treated with efflux pump inhibitors. S. Typhimurium ATCC 19585 and S. Typhimurium CCARM 8009 were highly susceptible to ciprofloxacin, erythromycin, levofloxacin, and sparfloxacin in the presence of phenylalanine-arginine-β-naphthylamide. The swimming motility of S. Typhimurium ATCC 19585 exposed to ceftriaxone was significantly reduced to 54% when compared to S. Typhimurium CCARM 8009 (93%). The numbers of attached S. Typhimurium CCARM 8009 cells were significantly increased by more than 1 log cfu/ml when exposed to ceftriaxone and ciprofloxacin. The relative gene expression was stable in S. Typhimurium CCARM 8009 in the presence of ceftriaxone and ciprofloxacin compared to the absence of antibiotics. These results suggest that the antibiotic susceptibility of S. Typhimurium having different antibiotic resistance profiles varied depending on the presence of ceftriaxone and ciprofloxacin.