Effect of Danofloxacin Treatment on the Development of Fluoroquinolone Resistance in in Calves.

is a leading cause of foodborne gastroenteritis. Recent studies have indicated a rise in fluoroquinolone-resistant (FQ-R) in cattle, where FQ is used to control bovine respiratory disease (BRD). To assess the effect of danofloxacin treatment on the development of FQ-resistance in , 30 commercial calves were divided into Group 1, Group 2, and Group 3 ( = 10), and were all inoculated orally with FQ-susceptible (FQ-S) ; seven days later, Group 3 was challenged with transtracheal , and one week later, Group 2 and Group 3 were injected subcutaneously with danofloxacin. Rectal feces were collected to determine relative percentages of FQ-R via culture. Before oral inoculation with , 87% of calves were naturally colonized by FQ-R . Two days after the inoculation, FQ-R decreased substantially in the majority of calves. Within 24 h of danofloxacin injection, almost all populations shifted to an FQ-R phenotype in both FQ-treated groups, which was only transitory, as FQ-S strains became predominant during later periods. Genotyping indicated that the spike seen in FQ-R populations following the injection was due mainly to enrichment of preexisting FQ-R , rather than development of de novo FQ resistance in susceptible strains. These results provide important insights into the dynamic changes of FQ-resistant in cattle in response to FQ treatment.