Soil-borne reservoirs of antibiotic-resistant bacteria are established following therapeutic treatment of dairy calves

We determined if antibiotics residues that are excreted from treated animals can contribute to persistence of resistant bacteria in agricultural environments. Administration of ceftiofur, a third-generation cephalosporin, resulted in a approximate to 3 log increase in ceftiofur-resistant Escherichia coli found in the faeces and pen soils by day 10 (P=0.005). This resistant population quickly subsided in faeces, but was sustained in the pen soil (approximate to 4.5 log bacteriag(-1)) throughout the trial (1 month). Florfenicol treatment resulted in a similar pattern although the loss of florfenicol-resistant E.coli was slower for faeces and remained stable at approximate to 6 log bacteriag(-1) in the soil. Calves were treated in pens where eGFP-labelled E.coli were present in the bedding (approximate to 2 logg(-1)) resulting in amplification of the eGFP E.coli population approximate to 2.1 log more than eGFP E.coli populations in pens with untreated calves (day 4; P<0.005). Excreted residues accounted for >10-fold greater contribution to the bedding reservoir compared with shedding of resistant bacteria in faeces. Treatment with therapeutic doses of ceftiofur or florfenicol resulted in 2-3 logg(-1) more bacteria than the estimated ID50 (2.83 CFUg(-1)), consistent with a soil-borne reservoir emerging after antibiotic treatment that can contribute to the long-term persistence of antibiotic resistance in animal agriculture.