Analysis of antibiotic resistance genes fate and its drivers during anaerobic digestion of animal manure

Abstract: Antibiotic resistance genes (ARGs) have been considered as an emerging pollutant. The occurrence of ARGs is mainly due to the overuse of antibiotics. China is the largest producer and consumer of antibiotics in the world, and it was estimated that about 162 000 t of antibiotics were used in China in 2013, among which 52% was used for animal breeding. However, after used, 30%-90% of these veterinary antibiotics were excreted with feces and urine for partially metabolized by animal body. Therefore, animal manure has been considered as an important reservoir of antibiotic residues and ARGs. If not treated effectively, ARGs can enter into other environmental mediums with land application. And ARGs can eventually enter into the human food chain which may endanger the public safety. Anaerobic digestion is a promising technology which can convert the organic matters to biogas and organic fertilizers. So it is widely used for animal manure disposal in different animal farms. Therefore, the variation of ARGs in anaerobic digestion system has big significance for reducing the risk of ARGs growth and transmission. The paper firstly showed that ARGs were ubiquitous in manure, water, soil, food and even in the air, on the basis of analyzing ARGs levels in different environmental mediums. About the detecting methods of ARGs, the main detecting methods contained PCR, real-time quantitative PCR (qPCR), high-throughput quantitative PCR and metagenomic technique, among which metagenomic technique was the most promising technology for comprehensive analyzing of ARGs in different types. About the variation of ARGs and its driving factors during anaerobic digestion, the paper analyzed the relating literatures at home and abroad. The results showed that the microbial community composition was the main driving factor of ARGs variation, which was due to the variation of the potential host microbes carrying ARGs during anaerobic digestion. Although many studies had investigated the ARGs and their potential hosts through the statistical method, the further confirmation of the real hosts for ARGs was also need. The determination of ARGs host microbes was the difficulty parts for the future research. Antibiotic residues and heavy metals were also the important driving factors for ARGs variation, so it is important to remove the antibiotic residues and heavy metals thoroughly to reduce the environmental risk of ARGs pollution. The mobile genetic elements played a key role in the horizontal transmission of ARGs, and they could be considered as some indicators for ARGs variation. On the whole, all the factors in anaerobic digestion system could directly or indirectly impact the ARGs variation. The operating parameters were the precondition for the whole system. Under the designed operating parameters, the microbial communities interacted with other physicochemical parameters in the anaerobic digestion system. The microbial communities affected the ARGs transmission by mobile genetic elements. Therefore, it was the research emphasis for future work to control ARGs by comprehensive regulation of different factors of anaerobic digestion system. The paper will provide some theoretical information for researchers about the variation, transmission and driving factors of ARGs in anaerobic digestion system with animal manure.