The role of excreted antibiotics in the establishment of persistent on-farm reservoirs of antibiotic-resistant bacteria
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Bioavailable antibiotic residues from treated animals likely contribute to the persistence of antibiotic-resistant bacteria in agricultural systems. To determine the magnitude of this effect, we evaluated changes in the number of antibiotic-resistant E. coli in the soil of calf pens after administration of therapeutic course of antibiotics. Antibiotic treatment resulted in a ~3-log (ceftiofur) and ~5-log (florfenicol) increase in the number of antibiotic-resistant E. coli in the pen soil and these resulting “reservoirs” remained stable for at least 20 days. To determine is antibiotic residues or fecal shedding of resistant bacteria is more important in this process, treated and untreated calves were introduced into pens that had been pre-sprayed with a GFP-labeled strain of E. coli. Results demonstrated that antibiotic residues were responsible for a 10-fold greater effect than fecal loading. We also estimated the “infectious dose 50” (ID50 = 2.83 log/g) for antibiotic-resistant E. coli within the first ten days of exposure (Chapter I). To determine if reservoirs of this magnitude exist under “real-world” conditions, we examined the density of soil-borne, antibiotic-resistant E. coli on 14 working farms in Washington State. A high proportion of soil samples from calf and heifer pens (32-61%) exceeded the predicted contact-dependent ID50 for transmission of antibiotic-resistant E. coli from soil to calves. Hospital pens (25-36%) also regularly exceeded this threshold, but this was much less common in lactating (1.5-3%), fresh (2-17%) and dry lot soils (0.8-6%). A one-year longitudinal study indicated that the abundance of antibiotic-resistant E. coli in soil is positively correlated with monthly average high and low temperature (r =0.58 to 0.7, P = 0.04 to 0.01). In a separate 16s-rDNA study, we observed a transient (10 days) microbiome change in the feces of animals after antibiotic treatment (P < 0.001), but no long-term treatment effects (P >0.05). Although pen soil community structure changed over time, if there were any antibiotic treatment effects these were masked by other factors (Chapter II). The presence of discrete and robust reservoirs of antibiotic-resistant E. coli in predictable locations presents a new opportunity for mitigating and reducing antibiotic resistance in food-animal production environments.