Quinolone resistance in veterinary isolates of Salmonella.

Twenty-seven nalidixic acid-resistant (MIC > or = 256 mg/L) isolates of salmonella from veterinary sources were also less susceptible to fluoroquinolones (range of MICs of ciprofloxacin, 0.12-2 mg/L). Six isolates were cross-resistant to one or more chemically unrelated antibacterial agents. The concentrations of enrofloxacin that inhibited DNA synthesis by 50% were similar to the MIC values for 23 of 27 isolates, suggesting a mutation in gyrA. Insertion of pNJR3-2 (gyrA) in nine of 20 isolates increased susceptibility to quinolones, suggesting that resistance was due to mutation in gyrA. Five of 27 isolates had reduced levels of accumulation of enrofloxacin. Two of the five also had increased susceptibility to quinolones when pNJR3-2 was introduced. None of the outer membrane protein profiles of the resistant isolates differed from those of sensitive control strains. Three of 27 isolates had differences in lipopolysaccharide profiles compared to control strains. Although the MIC of ciprofloxacin was less than the recommended UK break point concentrations for most isolates, the increasing incidence of quinolone-resistance in salmonella from veterinary sources is a matter of concern.