The presence of Haemophilus influenzae strains with low susceptibility to quinolones has been reported worldwide. However, the emergence and dissemination mechanisms remain unclear. In this study, a total of 14 quinolone-low-susceptible H. influenzae isolates were investigated phylogenetically and in vitro resistance transfer assay in order to elucidate the emergence and dissemination mechanisms. The phylogenetic analysis based on gyrA sequences showed that strains with the same sequence type determined by multilocus sequence typing were classified into different clusters, suggesting that H. influenzae quinolone resistance emerges not only by point mutation, but also by the horizontal transfer of mutated gyrA. Moreover, the in vitro resistance transfer assay confirmed the horizontal transfer of quinolone resistance and indicated an active role of extracellular DNA in the resistance transfer. Interestingly, the horizontal transfer of parC only occurred in those cells that harbored a GyrA with amino acid substitutions, suggesting a possible mechanism of quinolone resistance in clinical settings. Moreover, the uptake signal and uptake-signal-like sequences located downstream of the quinolone resistant-determining regions of gyrA and parC, respectively, contributed to the horizontal transfer of resistance in H. influenzae. Our study demonstrates that the quinolone resistance of H. influenzae could emerge due to the horizontal transfer of gyrA and parC via recognition of an uptake signal sequence or uptake-signal-like sequence. Since the presence of quinolone-low-susceptible H. influenzae with amino acid substitutions in GyrA have been increasing in recent years, it is necessary to focus our attention to the acquisition of further drug resistance in these isolates.
Keywords: Haemophilus influenzae; drug resistance mechanisms; quinolones.