The fate of antibiotics and antibiotic resistance genes in Large-Scale chicken farm Environments: Preliminary view of the performance of National veterinary Antimicrobial use reduction Action in Guangdong, China

In 2018, China implemented the Veterinary Antimicrobial Use Reduction Action to curb the rapid development of antibiotic resistance (AR). However, the AR-related pollutions in animal farms after the reduction policy has been poorly investigated. Here, we performed a comprehensive investigation combining UPLC-MS/MS, metagenomic, and bacterial genomic analyses in eight representative large-scale chicken farms in Guangdong, China. Our results showed that antibiotics and ARGs contaminations were more severe in broiler farms than in layer farms. Notably, diverse tet(X) variants were prevalent in the chicken farms. These tet(X)s was carried by diverse E. coli lineages and obviously correlated with ISCR2 and IS1B transposases. The resistomes in chicken farms was significantly correlated with microbial community, and multiple factor analyses indicated that the joint effect of antibiotics-microbial community-MGEs was the most dominant driver of ARGs. Host tracking identified a variety of ARG bacterial hosts and the co-occurrence of ARGs-MRGs-MGEs. Source tracking indicated that the inherent component represented the main feature of resistomes in different hosts, while ARG transfer between the chicken gut and farm environments were frequent. A multiperspective evaluation of AR risk revealed that the early effect of antibiotic reduction was exhibited by the mitigation of maximum level of risky ARGs, prevalence of environmental AR pathogens, and HGT potential of ARGs mediated by phage structures. Overall, our findings provide insights into the antibiotic and ARG profiles in large-scale chicken farms with different rearing strategies and demonstrate a preliminary view of the performance of antibiotic reduction actions in China.