Impact of fluoroquinolone resistance mutations on gonococcal fitness and in vivo selection for compensatory mutations

Background. Quinolone-resistant Neisseria gonorrhoeae (QRNG) arise from mutations in gyrA (intermediate resistance) or gyrA and parC (resistance). Here we tested the consequence of commonly isolated gyrA91/95 and parC86 mutations on gonococcal fitness.Methods.Mutant gyrA91/95 and parC86 alleles were introduced into wild-type gonococci or an isogenic mutant that is resistant to macrolides due to an mtrR-79 mutation. Wild-type and mutant bacteria were compared for growth in vitro and in competitive murine infection.Results.In vitro growth was reduced with increasing numbers of mutations. Interestingly, the gyrA91/95 mutation conferred an in vivo fitness benefit to wild-type and mtrR-79 mutant gonococci. The gyrA91/95, parC86 mutant, in contrast, showed a slight fitness defect in vivo, and the gyrA91/95, parC86, mtrR-79 mutant was markedly less fit relative to the parent strains. A ciprofloxacin-resistant (Cip ^R) mutant was selected during infection with the gyrA91/95, parC86, mtrR-79 mutant in which the mtrR-79 mutation was repaired and the gyrA91 mutation was altered. This in vivo-selected mutant grew as well as the wild-type strain in vitro.Conclusions.gyrA91/95 mutations may contribute to the spread of QRNG. Further acquisition of a parC86 mutation abrogates this fitness advantage; however, compensatory mutations can occur that restore in vivo fitness and maintain Cip ^R.