Effects Of Swine Manure Composting With Doxycycline Residue On Tetracycline Resistance Genes

Antibiotic resistance genes（ARGs）are considered as new emerging pollutants in the environment,which pose a major threat to global public health.Antibiotics are important factors affecting the ARGs in the composting process.Thus,aerobic composting experiments were conducted by collecting swine manure containing different concentrations of doxycycline.The aim of this study is to explore the effects of swine manure composting on tetracycline ARGs under doxycycline stress and to reveal its mechanism.The study provides a reference for reducing the pollution and spread of ARGs from livestock.Swine manure was used as the raw material and the sawdust was the conditioning agent.A small simulated composting was used for composting test.The experiment was divided into three groups:group CK（manure was derived from swine fed without any antibiotics）,group L（manure was derived from swine fed with doxycycline concentrations of 3 mg/kg body weight）,group H（manure was derived from swine fed with doxycycline concentrations of 5 mg/kg body weight）,with 3 replicates per group.The test period was 25 days.Samples were collecteded regularly to detect ARGs and MGEs,doxycycline concentrations,microbial community structure,physical and chemical indicators of compost.The results of our study were as follows:（1）Compost significantly reduced the absolute abundance of tet A,tet C,tet G,tet H,tet O,tet Q,tet W and cfr（P<0.05）.The removal rate of tet C,tet H,tet O,tet Q and tet W was above96%,and the removal rate of cfr was 81.93～99.99%.However,compost significantly increased the absolute abundance of tet X and int1 by 31907.13 and 5.12 times（P<0.05）.（2）At the end of composting,the concentrations of doxycycline in group L and group H was 58.82±1.82 mg/kg DM and 6.09±0.76 mg/kg DM,and their degradation rates were66.21±2.87%and 55.57±1.94%,respectively.The concentration of doxycycline was significantly correlated with the abundance of ARGs and MGEs during composting（P<0.05）.The removal efficiency of tet C,tet G,tet H,tet Q,tet W and cfr was reduced significantly under high concentration doxycycline of composting（P<0.05）,while the low concentration of doxycycline had no significant effect on the removal efficiency of ARGs and MGEs during composting.（3）At the end of composting,high concentrations of doxycycline changed the diversity of microbial community structure.The relative abundance of Firmicutes and Actinobacteria in group H was significantly higher than that in group CK（P<0.05）,and the relative abundance of Proteobacteria and Bacteroidetes in group H was significantly lower than that in group CK（P<0.05）.Correlation analysis showed that Georgenia is the potential host bacteria of tet A,Bacillus is the potential host bacteria of tet C,tet G and cfr,Firmicides is the potential host bacteria of tet O,Streptococcus and Lactobacillus are the potential host bacteria of tet Q and cfr,Luteimonas is the potential host bacteria of tet X.（4）The results of redundancy analysis showed that Euryarchaeota,Bacteroidetes,Spirochaetes,int1 and NO₃-N were significantly positively correlated with the tet A and tet G（P<0.05）.Firmicutes,temperature,EC,TN,TOC,C/N and NH₄-N were significantly positively correlated with the tet C,tet H,tet O,tet Q,tet W,and cfr（P<0.05）.Proteobacteria and p H was significantly positively correlated with tet X（P<0.05）.In summary,the decrease relative abundance of Firmicutes is the key factor for composting to remove tetracycline ARGs.The removal efficiency of tetracycline ARGs by composting was significantly reduced because of the increased abundance of Firmicutes and Actinobacteria under high concentrations of doxycycline in composting.Therefore,it is necessary to strictly control the use of antibiotics in livestock.It is beneficial to reduce the host bacteria of tetracycline ARGs,improving the removal efficiency of tetracycline ARGs by composting.