ALTERATIONS OF DNA TOPOISOMERASES OF RIEMERELLA ANATIPESTIFER WITH IN VITRO-REDUCED SUSCEPTIBILITY TO QUINOLONES

To elucidate on a potential association between mutations of topoisomerases and quinolone resistance in Riemerella anatipestifer, mutants of reduced quinolones susceptibility were generated from the four clinical isolates by serial passages in the presence of increased concentrations of nalidixic acid, ofloxacin, enrofloxacin and ciprofloxacin. Eleven alterations at eight positions (81, 82, 83, 84, 87, 117, 119, and 181) in GyrA and five different alterations at four (431, 452, 470, and 471) positions in GyrB were induced within mutants, resulting in various levels of resistance to different quinolones. The most frequent alteration was at Ser83 in GyrA, which concurred with that of previous clinical studies. To our best knowledge, alterations at Ala183 in GyrA and at Arg452 as well as Glu471 in GyrB were firstly reported. The study demonstrated that alterations of GyrA and GyrB are the prior mechanisms by which R. anatipestifer develops its resistance to quinolones. Expanding the quinolone-resistance determining region for R. anatipestifer is suggested as well.