A novel gyrB mutation in a fluoroquinolone-resistant clinical isolate of Salmonella typhimurium.

In order to study the role of gyrB in antibiotic resistance in post-ciprofloxacin therapy fluoroquinolone-resistant clinical isolates of Salmonella typhimurium, plasmid pBP548, which contains the Escherichia coli gyrB gene, was used in complementation studies. In a heterodiploid strain, the wild-type (quinolone sensitive) allele is dominant over the resistant allele therefore, eleven clinical isolates were complemented with gyrB encoded on pBP548. Only one transformant, L18pBP548, exhibited increased susceptibility to the quinolones nalidixic acid, ciprofloxacin and sparfloxacin. The amino acid sequence of the gyrase B protein from a wild-type and the pre-therapy S. typhimurium (deduced from the nucleotide sequence) was identical to that of E. coli from codons 436 to 470; however, a point mutation was identified in codon 463 of gyrB of the quinolone-resistant post-therapy isolate L18, giving rise to an amino acid substitution of serine to tyrosine.