Fluoroquinolone-resistant Brucella melitensis mutants obtained in vitro.

Three fluoroquinolone-resistant Brucella melitensis mutants were obtained after successive passages on norfloxacin-containing agar plates. The original strain was B. melitensis biovar 2 ATCC 23457. Mutants emerged at frequencies ranging between 1 x 10(-6) and 3 x 10(-8). The three mutants showed a gyrA Ala71-->Ser mutation as well as insertion of Ala340. No mutations were observed in the parC and gyrB genes. Minimum inhibitory concentrations (MICs) increased by four- to eight-fold from the wild-type to the first-step mutant for norfloxacin, ciprofloxacin, levofloxacin and moxifloxacin. MICs of the second-step mutant increased significantly only for norfloxacin and those for the third-step mutant for norfloxacin and ciprofloxacin, suggesting the involvement of some efflux mechanism.