Metagenomic insights into the influence of mobile genetic elements on ARGs along typical wastewater treatment system on pig farms in China

The wastewater treatment processes (WTP) on pig farms are heavily contaminated by antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) play an important role in shaping ARG profiles. Here we first employed metagenomic sequencing to follow the diversities and shifts of ARG associated mobile genetic elements (AAMGEs) including insertion sequences (ISs) and plasmids along the WTP for three pig farms in southeast China. The IS average relative abundance rose from the initial pig feces source to the wastewater storage lagoon (WSL) but decreased in the influent and rose in the effluent of the anaerobic digestor (AD). In contrast, plasmids were eliminated rapidly along this process. These results indicated that the AD reduced plasmid copies while IS abundance increased. We found a great diversity ISs, including IS91, ISNCY, IS630 and IS701, were large contributors to the transfer of multi-drug resistance. In addition, the tetracycline resistance genes co-occurred with a greater diversity of ISs than other ARG classes and this likely contributed to the high abundance of tetracycline resistance genes we found. The transfer of ARGs mediated by MGEs along the WTP of pig farms was a key contributor for the ARGs persistence in the environment of pig farms. Collectively, our findings demonstrated different fates for ISs and plasmids along the WTP for pig farms and suggested that AAMGE monitoring served as an important role in controlling ARGs in pig waste.