| Summary: | Fluoroquinolone (FQ) resistance in a major foodborne bacterial pathogen, <i>Campylobacter jejuni</i>, derived from cattle has recently become prevalent and poses a significant public health concern. However, the underlying factors for this increase are not entirely clear. To evaluate the effect of enrofloxacin treatment on FQ-resistance development in <i>C. jejuni</i>, 35 commercial calves were equally divided into five groups (Groups 1–5) and were orally inoculated with FQ-susceptible (FQ-S) <i>C. jejuni</i>. Eight days later, Groups 4 and 5 were challenged with <i>Mannheimia haemolytica</i> via a transtracheal route to induce a respiratory disease; after 8 days, Groups 2, 3, 4, and 5 were injected subcutaneously with enrofloxacin (7.5 mg/kg for Groups 2 and 4, and 12.5 mg/kg for Groups 3 and 5). Colonization levels by FQ-resistant (FQ-R) and FQ-S <i>Campylobacter</i> in rectal feces were determined via differential culture throughout the experiment. Before oral inoculation with <i>C. jejuni</i>, only five calves were naturally colonized by <i>Campylobacter</i>, four of which were also colonized by FQ-R <i>C. jejuni</i> (three in Group 1 and one in Group 3). Soon after the oral inoculation, almost all calves in the groups became stably colonized by FQ-S <i>C. jejuni</i> (~3–6 log<sub>10</sub> CFU/g), except that the four calves that were pre-colonized before inoculation remained positive with both FQ-R and FQ-S <i>C. jejuni</i>. Following enrofloxacin administration, <i>C. jejuni</i> colonization declined sharply and rapidly in all treated groups to undetectable levels; however, the vast majority of the animals were recolonized by <i>C. jejuni</i> at comparable levels 72 h after the treatment. Notably, no FQ-R <i>C. jejuni</i> was detected in any of the calves that received enrofloxacin, regardless of the drug dose used or disease status of the animals. The lack of detection of FQ-R <i>C. jejuni</i> was likely due to the localized high concentration of the antibiotic in the intestine, which may have prevented the emergence of the FQ-R mutant. These findings indicate that single-dose enrofloxacin use in cattle poses a low risk for selection of de novo FQ-R mutants in <i>C. jejuni</i>.
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