A NOVEL GYRB MUTATION IN A FLUOROQUINOLONE-RESISTANT CLINICAL ISOLATEOF SALMONELLA-TYPHIMURIUM

In order to study the role of gyrB in antibiotic resistance in post-ci profloxacin therapy fluoroquinolone-resistant clinical isolates of Sal monella typhimurium, plasmid pBP548, which contains the Escherichia co li gyrB gene, was used in complementation studies. In a heterodiploid strain, the wild-type (quinolone sensitive) allele is dominant over th e resistant allele therefore, eleven clinical isolates were complement ed with gyrB encoded on pBP548. Only one transformant, L18pBP548, exhi bited increased susceptibility to the quinolones nalidixic acid, cipro floxacin and sparfloxacin. The amino acid sequence of the gyrase B pro tein 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.