Dissemination of antibiotic resistance genes (ARGs) by rainfall on a cyclic economic breeding livestock farm

Livestock farms are a propagating source of antibiotic resistance genes (ARGs) via the discharge of animal waste carrying resistant bacteria. Groundwater is widely used in traditional livestock farming. However, there is little available information on ARGs in pumped well water. To investigate the dissemination of ARGs in well water and to assess the impact of rainfall on ARGs, the abundance and diversity of ARGs on a long-term co-livestock feedlot farm where both pig and fish farming is conducted were analysed during the dry and wet seasons in this study, and twenty ARGs were detected. The total concentrations of the target ARGs (∑ARGs) in piggery wastewater and fishponds were approximately 109 copies/mL and 106 copies/mL, respectively. The wastewater treatment plant at this farm failed to avoid ARG contamination, and the biogas digester and lagoon treatment showed low abilities to eliminate ∑ARGs. The open operation mode of the lagoon and fishpond facilitated the dissemination of ARGs, which was assisted by the nearby stream, especially in the wet season. Additionally, the tetracycline resistance gene tetB showed the highest levels among the target ARGs in well water, which were higher in the wet season than the dry season. Affected by rainfall, the concentrations of ARGs in the samples rose, and the correlation between surface water and groundwater increased. These results suggested that both the surface water and the well water in the livestock farming area harboured serious ARGs contamination, which could pose ecological risks and cause food safety problems.