MUTATION IN DNA GYRASE OF NORFLOXACIN-RESISTANT CLINICAL ISOLATES OF NEISSERIA-GONORRHOEAE

Background and Objectives: Recently a rapid decrease in the susceptibi lity of Neisseria gonorrhoeae isolates to fluoroquinolones has occurre d and gonococcal fluoroquinolone resistance is now a significant probl em in the treatment of gonorrhoea in Japan. Thus, in order to investig ate the quinolone resistance mechanisms in clinical isolates of N gono rrhoeae we studied an alteration in the DNA gyrase subunit A (GyrA) wh ich is well-known as a common mechanism of bacterial quinolone resista nce. Materials and methods: Four clinical isolates of N gonorrhoeae re sistant to norfloxacin and 5 strains susceptible to norfloxacin, inclu ding 2 clinical isolates and 3 WHO reference strains, were tested in t his study. To identify mutations in the GyrA genes of gonococcal strai ns, poly merase chain reaction and direct DNA sequencing were performe d. Results: A single base change (serine codon TCC changed to phenylal anine codon TTC), which resulted in an amino acid change in GyrA at po sition 91, was identified in all 4 norfloxacin-resistant strains for w hich the MICs of norfloxacin ranged from 1.0 to 8.0 mu g/ml, while no mutation within GyrA was detected in 5 norfloxacin-susceptible strains for which the MICs of norfloxacin ranged from 0.004 to 0.063 mu g/ml. Conclusions: The results from this study suggest that the serine-91 t o phenylalanine substitution in GyrA is probably an essential mutation in fluoroquinolone resistance in clinical isolates of N gonorrhoeae.