Mechanisms responsible for the maintenance of high prevalence of antimicrobial drug-resistant Escherichia coli in dairy calves

The prevalence of antimicrobial drug resistant bacteria is typically highest in younger animals, and the resistance traits can theoretically be maintained in a population through (a) direct antimicrobial selective pressure, (b) secondary advantage conferred by the resistance genes in the absence of antimicrobial selective pressure, and (c) linkage to other selectively advantageous genes. We assessed the prevalence of antimicrobial resistant Escherichia coli at a Holstein dairy farm and confirmed a higher prevalence of tetracycline resistance in younger calves. The most common phenotype among calf isolates included resistance to streptomycin, sulfa-drugs and tetracycline (SSuT), which was most prevalent during the period when calves were on milk diet. We inoculated animals with strains of SSuT and susceptible E. coli and found a higher frequency of SSuT strains in neonatal calves (P < 0.001), but no difference in older animals (P = 0.5). We also fed calves a milk supplement with and without oxytetracycline and found that calves receiving the oxytetracycline had slightly lower prevalence of tetracycline resistant and SSuT E. coli. These data indicate that direct selection was not critical to maintain SSuT strains at the dairy.; We then inoculated newborn calves with cured-SSuT (SSuT strains cured from antimicrobial resistance genes) and progenitor SSuT strains. On average the cured-SSuT strains were as competitive as the progenitor strains indicating that any selective advantage was not a secondary by-product of the resistance genes themselves. We observed from the above studies that there might be a relationship between the presence of SSuT E. coli and a milk supplement. Using a feeding trial we demonstrated that milk supplement selected for significantly more SSuT E. coli in calves compared with the no-supplement control group (≈ 40% vs. ≈ 20% respectively). The presence of oxytetracycline in the milk supplement did not add to this effect. In vitro studies demonstrated that the milk supplement supported a significantly higher cell density of SSuT strains compared with other strains. From these results we speculate that the SSuT resistance genes are genetically linked to gene(s) that confer a selective advantage in the presence of milk supplement.