Quinolone-resistant determining region (QRDR) of DNA phrase gyrA genes from Salmonella typhi 275 (a clinically isolated quinolone susceptible strain) and its spontaneous quinolone resistant mutant S.typhi RG1 were sequenced and the interaction between quinolones and DNA gyrase was analyzed. The results revealed that the 128-434 nucleotide sequence of S. typhi 275 gyrA were 92.51%

97.72% and 77.35% identical with those of Escherichia colt KL-16

S. typhimirum NCTC 74 and Pseudomonas aeruginosa PAO1

which contributed to 3、2 and 13 differences

respectively

in the deduced amino acids in each bacteria. In comparision with S. typhi 275

a single mutation was found at base 247 in gyrA of S. typhi RGI

which changed from T to G and led to an amino acid substitution of Ser83Ala. The mutation might be resposible for the increase of MICs of nalidixic acid

ofloxacin and ciprofloxacin on S.typhi from 2

0.06

<0.03 to 512

2

1mg/L

respectively. These results indicated that the antibacterial mechanism of quinolone on S. typhi was the same as those on the other bacteria and that the mutation in QRDR of gyrA was also the major reason for quinolone resistance in S. typhi.