Impacts of a decade of annual tylosin, chlortetracycline and sulfamethazine application on drug persistence, bacterial hylogenetic diversity and functional diversity in an agricultural soil

Use of antibiotics as growth promoting agents in livestock production contributes to the increasingly worrisome development of antibiotic resistance . In order to evaluate the long term impacts of antibiotic exposure on soil microbial populations, a series of field plots were established in London, Ontario, Canada in 1999 that have since received annual applications of a mixture of sulfamethazine, tylosin and chlortetracycline at concentrations (0, 0 . 1, 1 . 0 and 10 mg/kg soil) bracketing that which would result from an annual application of manure from medicated swine . Following ten annual applications, biodegradation potential and persistence of these drugs was evaluated . Residues of sulfamethazine and tylosin, but not chlortetracycline, were removed much more rapidly in soil with a history of exposure to the drugs than in untreated control soil . Residues of 14 C-sulfamethazine were rapidly and thoroughly mineralized to 14 CO 2 in the historically treated soils, but not at all in the untreated soil . Enrichment cultures of bacteria able to degrade sulfamethazine were obtained from historically treated soils, but not from untreated soil. The abundance of viable bacteria and their relative distribution in major bacterial phylogenetic groups of bacteria was evaluated microscopically. Using DAPI and the Molecular Probes Live/Dead stain, there was no treatment effect on the abundance of viable bacteria . There were no differences between treatments with respect to the relative abundance of Alpha- Proteobacteria, Beta- Proteobacteria, Gamma- Proteobacteria, low-GC and high-GC Gram positive bacteria . There were differences in the abundance of Planctomycetes and the Cytophaga-Flavobacterium cluster with treatment, but these did not indicate a coherent dose-response. Long-term treatment altered functional microbial populations as detected using the pollution induced community tolerance (PICT) test evaluated with either each of the antibiotics present individually, or as a mixture . Overall, these results indicate that soil bacteria adapt functionally to long-term exposure to some veterinary antibiotics, notably resulting in sharply reduced persistence of the drugs . Accelerated biodegradation of antibiotics in matrices exposed to agricultural, wastewater, or pharmaceutical manufacturing effluents would attenuate environmental exposure to antibiotics, and merits investigation in the context of assessing potential risks of antibiotic resistance development in the environment.