Emergence of fluoroquinolone resistance in the native Campylobacter coli population of pigs exposed to enrofloxacin.

Objective: The effect of a single 5 day enrofloxacin treatment on the native Campylobacter coli population in conventionally weaned 5-week-old pigs was investigated.

Materials: Twelve pigs were split into two groups of six: one group was treated with a therapeutic dose (15 mg/pig/day) of enrofloxacin and the other remained untreated to act as the control. Campylobacter coli were isolated from faecal samples and tested for ciprofloxacin resistance by measuring MIC values. Mutations in the quinolone resistance-determining region (QRDR) of the gyrA gene of resistant isolates were identified by sequencing and denaturing HPLC. Levels of enrofloxacin and its primary metabolite ciprofloxacin in the pig faeces were also measured by HPLC.

Results: No quinolone-resistant C. coli (n = 867) were detected in any of the pigs prior to treatment, indicating <0.1% resistance in the group. Resistant C. coli were isolated from pigs for up to 35 days after treatment with a therapeutic dose. These resistant C. coli had MIC values of 128 mg/L and 8-16 mg/L for nalidixic acid and ciprofloxacin, respectively, and the same single point mutation causing a Thr-86 to Ile substitution in the QRDR was identified in each. The concentration of enrofloxacin in the pig faeces was 2-4 micro g/g faeces for the duration of the 5 day therapeutic treatment and was detected up to 10 days post-treatment. Ciprofloxacin was also measured and peaked at 0.6 micro g/g faeces in the treated group.

Conclusion: This study provides evidence that a single course of enrofloxacin treatment contributes directly to the emergence and persistence of fluoroquinolone resistance in C. coli.