title: Role of calf-adapted Escherichia coli in maintenance of antimicrobial drug resistance in dairy calves.
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 microg/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P = 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 microg/ml), sulfadiazine (>512 microg/ml), and tetracycline (>4 microg/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.
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington 99164, USA.
authors.forename: Artashes R
authors.initials: AR
authors.lastname: Khachatryan
authors.forename: Dale D
authors.initials: DD
authors.lastname: Hancock
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Besser
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Call
mesh_terms: D000375:Aging / Q000502:physiology*; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology*; D003612:Dairying; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000187:drug effects*; D004927:Escherichia coli Infections / Q000382:microbiology / Q000662:veterinary*; D008826:Microbial Sensitivity Tests; D010118:Oxytetracycline / Q000494:pharmacology; D015995:Prevalence
chemical_list: D000900:Anti-Bacterial Agents; D010118:Oxytetracycline