Studying the effect of administration route and treatment dose on the selection of enrofloxacin resistance in commensal Escherichia coli in broilers

Objectives: Factors potentially contributing to fluoroquinolone resistance selection in commensal Escherichia coli strains in poultry were studied through a series of in vivo experiments. The effect of the initial prevalence of enrofloxacin resistance in the E. coli gut microbiota, effect of the bacterial fitness of the enrofloxacin-resistant strain and effect of treatment with enrofloxacin (effect of dose and effect of route of administration) were assessed. Methods: Four in vivo studies with broiler chickens were performed. Right after hatching, the chicks were inoculated with either a bacteriologically fit or a bacteriologically non-fit fluoroquinolone-resistant strain as either a minority or the majority of the total E. coli population. Six days later, the chicks were treated for three consecutive days either orally or parenterally and using three different doses (under-, correct- and over-dose) of enrofloxacin. The faecal shedding of E. coli strains was quantified by plating on agar plates either supplemented or not supplemented with enrofloxacin. Linear mixed models were used to assess the effect of the aforementioned variables on the selection of enrofloxacin resistance. Results: The factors that significantly contributed were treatment (P < 0.001), bacterial fitness of the resistant donor strain (P < 0.001), administration route (P = 0.052) and interactions between bacterial fitness and administration route (P < 0.001). Conclusions: In the currently used models, fluoroquinolone resistance selection was influenced by treatment, bacterial fitness of the inoculation strain and administration route. The use of oral treatment seems to select more for fluoroquinolone resistance, particularly in the model where a non-fit strain was used for inoculation