Evidence for Accelerated Biodegradation of Antibiotics in Soil Following Repeated Exposure in the Field.

Antibiotics can be entrained into soil via fertilization with manures or biosolids that contain excreted drug residues. In order to evaluate the fate and effects of antibiotics in soil, we initiated in 1999 on our research farm in London, ON a long term field experiment. Every subsequent spring, a series of small plots have received an annual spring application of a mixture of tylosin (TYL), sulfamethazine (SMZ) and chlortetracycline (CTC) to achieve concentrations (mg/kg soil) of 0.1, 1, or 10. In 2010 several more plots were established that received an annual spring application of a mixture of erythromycin (ERY), clarithromycin (CLA) and azithromycin (AZI) to reach concentrations of 0.1 or 10 mg/kg soil. Control plots are maintained that do not receive antibiotics, and all plots were continuously cropped to soybeans. The soil persistence and dissipation pathways of the antibiotics were evaluated in the laboratory using radioisotope methods and HPLC-MS. Compared to soil with no history of drug exposure, the persistence of TYL, SMZ, ERY and CLA was far shorter in soils that were exposed to the antibiotics in the field. ¹⁴C-labelled SMZ, ERY and CLA were rapidly and thoroughly mineralized by the historically exposed soils, whereas they were much more slowly in the unexposed control soil. Enhanced degradation of ERY and CLA was established with exposure to both 0.1 and 10 mg/kg. Enhanced degradation of SMZ was detected in soils exposed to 10 mg/kg, but not lower concentrations. Overall, these results are consistent with enhanced biodegradation of several of antibiotics that vary widely in structure. The relationship between abundance of SMZ-degrading bacteria, SMZ concentration, and mineralization of the drug was explored in laboratory incubations with an SMZ-degrading bacterium, Microbacterium sp. Strain C448. There was insignificant mineralization of ¹⁴C-SMZ in uninoculated soil amended with SMZ concentrations from 0.01 to 10 µg/g. In soil inoculated with 10⁷ viable C448/g, ¹⁴C-SMZ was mineralized with comparable kinetics regardless of the drug concentration. However, in soil inoculated with 10⁴ viable C448/g, there was very little mineralization at concentrations of 0.01 to 1 µg/g SMZ, whereas at a concentration of 10 µg/g, the rate of ¹⁴CO₂ accumulation was very significant. We hypothesize that drug concentrations sufficient to support growth in soil is required for the development of a biodegrading flora with repeated exposure.