Revised Title: Detection of tetracyclines and tetracycline resistant bacteria in soils under long-term swine effluent

These days, there are increasing concerns that the use of veterinary antibiotics could enhance persistence of antibiotic resistant bacteria in the environment, and consequently constitute a significant risk to the human health as well as animal well-being. Tetracyclines account for more than 50% of all antibiotics used in the swine industry. Unfortunately, only limited scientific evidence is available connecting the fate of antibiotics in the soil following animal waste application and the development of bacterial resistance. In the present study we quantify tetracyclines in 36 agricultural soils that have been continuously fertilized with swine effluent (50, 150, and 450 kg N ha -1 ) for more than 15 years, and assess the level and occurrence of tetracycline resistant bacteria in these soils. Residue chlortetracycline concentrations were detected in the soils at all three swine effluent application rates. However, the presence of chlortetracycline had virtually no effect on the development of tetracycline resistance in bacteria isolated from these soils. Based on the testing of more than 3,000 soil bacteria isolates, we found no significant increase in the occurrence and level of chlortetracycline susceptible bacteria in the fertilized soil. To account for a possible transfer of tetracycline resistant bacteria from the swine effluent to soils, two commonly found tetracycline resistant genes were analyzed in the swine effluent and fertilized soils. While both genes were present in the swine effluent, they were not detectable in the swine effluent applied soils. Our findings suggest that while antibiotic resistant bacteria could potentially be added to the agricultural soils along with swine effluent, it would not necessarily lead to the development of antibiotic resistance of the bacteria originally present in the soil.