Unexpected dose response of repeated annual veterinary drug applications on microbial diversity and functions in an agricultural soil

Antibiotics are widely used in North American and Asian livestock and poultry production for growth promotion, prophylaxis and treatment of illness. There is a widespread concern that this practice promotes resistance and erodes the efficacy of antibiotics used in human medicine, a crucially important public health challenge. It is unknown if concentrations of antibiotics released through livestock production are sufficient to promote resistance in environmental bacteria. A reservoir of antibiotic resistance genes in the environment that is made larger through contamination with agricultural wastes may represent an enhanced threat to human health. Within this context we have been exploring the long-term impacts of selected veterinary antibiotics on soil microbial populations. A series of replicated plots were established in London, Ontario, Canada in 1999 that have since been treated once annually every spring with a mixture of sulfamethazine, tylosin and chlortetracycline. The drug mixture was applied to achieve a range of soil concentrations (0, 0.1, 1.0, 10.0 mg of each drug per kg soil) encompassing that which would be expected from a typical rate of annual application of manure from a commercial swine farm. The antibiotics were added to the soil without the addition of manure in order to directly evaluate the impacts of the drugs on soil microorganisms without a confounding effect from the enteric flora shed by animals.