Role of Calf-Adapted Escherichia coli in Maintenance of Antimicrobial Drug Resistance in Dairy Calves

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Abstract
The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli . Isolates resistant to tetracycline (>4 μg/ml) were more prevalent in 12 μg/ml), sulfadiazine (>512 μg/ml), and tetracycline (>4 μg/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli . Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves ( P < 0.001), whereas there was no difference in older animals ( P = 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.