RESUMEN
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 â¼ 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 (â¼ 4.5 log bacteria g(-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 â¼ 6 log bacteria g(-1) in the soil. Calves were treated in pens where eGFP-labelled E. coli were present in the bedding (â¼ 2 log g(-1)) resulting in amplification of the eGFP E. coli population â¼ 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 log g(-1) more bacteria than the estimated ID50 (2.83 CFU g(-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.