Characterisation of Salmonella spp. mutants produced by exposure to various fluoroquinolones

We repeatedly exposed six Salmonella spp. strains of different serotypes (three susceptible to nalidixic acid and three resistant) to constant low concentrations of various fluoroquinolones with the aim of characterising the mutations that produce the first decrease in susceptibility to these agents. The fluoroquinolone-susceptibility of all the strains was reduced after repeated exposure to these agents. However, gyrA mutants were not always produced. Furthermore, the type of mutation produced and the time taken for it to appear varied depending on the initial resistance to nalidixic acid, the antibiotic used and the serotype involved. Therefore, we believe that the initial decrease in quinolone-susceptibility is due to various mechanisms and, in many cases, is not caused by mutations in the gyrA gene