Role of novel gyrA mutations in the suppression of the fluoroquinolone resistance genotype of vaccine strain Salmonella Typhimurium vacT (gyrA D87G)

Objectives: This study was aimed at characterizing the gyrA locus and determining its impact on fluoroquinolone susceptibility, DNA supercoiling degree and growth rate of Salmonella Typhimurium live vaccine strain vacT in comparison with its parent M415. Furthermore, the role of multiple drug resistance efflux in the susceptibility of vacT to fluoroquinolones and macrolides was investigated. Methods: DNA sequences were determined for genes gyrA, gyrB, parC and parE of M415 and three consecutive mutants Nal2ori, Nal2passage and vacT. The impact of gyrA mutations on the fluoroquinolone susceptibilities and relative DNA supercoiling degrees was investigated by a complementation assay using wild-type gyrA (gyrA(+)) and a reporter gene system, respectively. Doubling times of the strains and MICs of different antibiotics in the absence and presence of an efflux pump inhibitor (EPI) were determined. Results: Besides the gyrA mutation D87G, two novel mutations (G75A and A866S) were identified in the three mutants and a third novel mutation W59R in vacT. Fluoroquinolone susceptibilities and DNA supercoiling degrees of all three mutants were reduced compared with those of M415. Introduction of the gyrA(+) allele restored fluoroquinolone susceptibilities of the two intermediate strains to the wild-type level; however, for vacT, MICs of fluoroquinolones were reduced below those of M415. VacT had a higher susceptibility to macrolides and the EPI compared with M415. Conclusions: The data point to a combination of at least one non-gyrA mutation and novel gyrA mutation(s) as the basis for the unusual fluoroquinolone susceptibility of vacT.