Tuesday, November 14, 2006 - 12:50 PM

Use of Antibiotics in Animal Agriculture and the Fate of Antibiotic Residues and Resistance Genes in the Environment after Land Application of Swine Manure.

Roderick I. Mackie, Univ of IL-Urbana Champaign, Dept of Animal Sciences, 458 Animal Sciences Lab, Urbana, IL 61801

Antibiotics are widely used in animal agriculture. It is estimated that ca.75% of antibiotics are not absorbed by animals and are excreted in urine and feces. This extensive use of antibiotics has resulted in antibiotic resistance in many intestinal bacteria, which are also excreted in feces, resulting in dissemination of resistance genes into waste holding environments. Animal waste products are generally stored before disposal into the environment. Land application of animal waste is the most common disposal method in the US. To assess the impact of manure management on groundwater quality, groundwater samples were collected near confinement facilities that use lagoons for manure storage and treatment. Although tetracyclines were regularly used at both facilities, they were infrequently detected in manure samples and then at relatively trace concentrations.  Fewer than 5% of the groundwater samples collected at either of the facilities contained detectable levels of these antibiotics even in samples from wells that had been significantly impacted by manure seepage. Bacterial tetracycline resistance genes serve as distinct genotypic markers to indicate the dissemination and mobility of antibiotic resistance genes that originate from the lagoons.  PCR analysis of genomic DNA extracted from the lagoon and groundwater samples, detected four commonly occurring tetracycline (tet) resistance genes. The detection frequency of tet genes was much higher in wells located closer to and down-gradient from the lagoons than in wells more distant from the lagoons. These results suggested that in the groundwater underlying both facilities, tetracycline resistance genes exist and are somewhat persistent, but that the distribution and potentially the flux for each tet gene varied throughout the study period.