Genotypic evolution in a quinolone-resistant Neisseria gonorrhoeae isolate from a patient with clinical failure of levofloxacin treatment.

Recently, a reduction in the antimicrobial susceptibility of clinical isolates of Neisseria gonorrhoeae to newer fluoroquinolones including levofloxacin in vitro has been recognized in Japan. We examined the quinolone resistance mechanisms in N. gonorrhoeae isolates from a patient with clinical failure of levofloxacin treatment. Man with gonococcal urethritis was treated with oral 100 mg levofloxacin 3 times daily for 7 days. However, clinical failure of the treatment was observed. The minimum inhibitory concentration of levofloxacin for the posttreatment isolate (4.0 microg/ml) was 4-fold higher than that for the pretreatment isolate (1.0 microg/ml). To analyze quinolone resistance mechanisms in the set of isolates, we performed DNA sequencing of the quinolone resistance-determining regions within the gyrA and parC genes. Moreover, we assayed the intracellular levofloxacin and norfloxacin accumulation level in these isolates. The pretreatment isolate contained three substitutions compared to susceptible wild-type isolate, including serine to phenylalanine at position 91 and aspartic acid to asparagine at position 95 in the GyrA protein, and serine to proline at position 88 in the ParC protein. The posttreatment isolate had four substitutions, including the same three substitutions and an additional glutamic acid to glutamine substitution at position 91 in ParC. There was no significant difference in the level of accumulation of levofloxacin and norfloxacin between the pretreatment and posttreatment isolates. Our results indicate that levofloxacin selects a mutant having an additional alteration within the gene cording for the ParC protein during treatment, which may have enhanced quinolone resistance in the organism.