Title: Livestock feeds as a means of dissemination of antimicrobial resistant organisms
Principal Investigator: Dale Hancock
Organization: Washington State University
Description:
Hypotheses: Livestock feeds play an important role in the dissemination of antimicrobial resistant bacteria and resistance genes.
Aim 1. To determine the frequency with which bacteria carrying different resistant phenotypes and selected genotypes are found in feeds and the ability of these bacteria to multiply using feed as a substrate.
Rationale. We propose a sample size that will allow us to accurately estimate the frequency with which E coli, Enteroccoci, and S. enterica bearing different resistance phenotypes occur in feed. Isolates will be further characterized to determine the genetic nature of resistance: whether resistance genes reside on conjugative or mobilizable plasmids and, for selected antimicrobials, the specific genotype that accounts for resistance. The inclusion of a large number of samples collected at feed mills will allow us to provide estimates for feeds in commerce. Notably this can include a subset estimate of the frequency of different resistance phenotypes and genotypes disseminated in feeds over great distances, since some feeds in the northwestern United States can confidently be assumed to have come from other regions (e.g., cotton seed and soybean products). The feeds collected at farms are necessary for aim 2, but they will also contribute to the overall estimate of the frequencies of resistance phenotypes and genotypes and to the breakdown by feed category.
Aim 2. To determine, in farm (observational) settings, if resistant E. coli, Enteroccoci, and salmonellae found in feeds become part of the intestinal flora of cattle consuming tile feeds.
Rationale. Studies of E coli biotypes in cattle have found that new subtypes, previously not observed, are sometimes shed by multiple members of a group, suggesting feed as a source (Hinton). Studies in humans have suggested that foods represent a continuing source for commensal E coli that are Involved in the continuing turnover of phenotypes that is commonly observed in humans. A number of studies have reported fecal shedding of S. enterica strains found in feeds that cattle are consuming. The frequency with which this occurs is not known, however, and it is also not known whether this is an important route for introduction of new resistance genes into a herd. We have proposed to intensively (weekly) monitor dairy farms because the average commercial dairy receives several hundred loads of feed per year. In light of our pilot data showing that detectable E. coli can be found in approximately one-third of feed samples (and we suspect a similar prevalence for Enteroccoci) and that a substantial fraction of these harbor resistance genes, we believe the hypothesis will be tested.
Aim 3. To determine, in an experimental setting, if antimicrobial resistant E.coli strains contaminating feed are able to colonize cattle, if transfer of resistance genes to commensal E. coli occurs, and if the treatment of challenged cattle with antimicrobials influences the probability of colonization or transfer of resistance genes.
Rationale. Calves challenged with feed-borne E. coli, which are resistant to extended spectrum beta lactams (ESBL) will be followed overtime and fecal isolates typed. Treatments will include a group of calves given ESBL, a group of calves given chlorotet, and a group not receiving any antimicrobial. We will determine if challenge strains can be found in feces after challenge and for 2 weeks after, and if transfer of ESBL resistance gene occurs in other bacterial flora.
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