Effects Of Different Swine Manure To Wheat Straw Ratios On Antibiotic Resistance Genes And The Microbial Community Structure During Anaerobic Digestion

Antibiotic resistance gene?ARGs?as a new type of pollutant,which could be transported by vertical and horizontal transfer in microbial community.In this way,environmental bacteria can obtain ARGs,including human pathogen bacteria.Currently,antibiotics are used widely in intensive animal husbandry to prevent diseases and promote growth,thus,animal manure is an important reservoir of antibiotics and ARGs.Anaerobic digestion?AD?is an effective and economic approach for manure management because it can convert organic solid waste into fuel gas and fertilizer,however,AD residues are a reservoir of ARGs.Thus,their application to agricultural soils is assumed to increase ARGs and select for resistant bacterial populations in soils.The objectives of this study were to investigate the variations in ARGs during AD using three different proportions of swine manure relative to wheat straw,and the bacterial community,environmental factors?SCOD,VFAs,pH,and NH4+?to explore the underlying mechanisms responsible for the variation in ARGs.The main results and conclusions are as follows:During the 0¹7 days,the order of biogas accumulation was C3?7:3?> C2?5:5?> C1?3:7?.With the AD,the biogas accumulation of C2 increased significantly,and the total biogas production was 1711,3857 and 3225 mL after AD,respectively.Therefore,the highest accumulation of biogas can be obtained when the ratio of pig manure and straw is 5:5?C2 treatment?.The value of acid produced in volatile fatty acids?VFAs?is acetic acid> propionic acid> butyric acid> isovaleric acid> n–valeric acid> isobutyric acid.The total VFAs significantly increased on 25 days,but the total concentration of VFAs decreased after 25 days?especially C2 and C3 treatment?,thus the concentration of VFAs decreased in the three treatments after AD.Five tetracycline resistance genes?tetC,tetG,tetQ,tetW,and tetX?,three sulfonamide resistance genes?sul1,sul2,and dfrA7?,four macrolide resistance genes?ermB,ermF,ermQ,and ermX?,and the integrase gene of class 1 integrons?intI1?were detected by standard PCR.After AD,the total relative abundance of ARGs decreased,and compared with the tetracycline resistance genes?tet genes?and macrolide resistance genes?erm genes?,the AAs of sulfonamide resistance genes?sul genes?exhibited greater decreases.Overall,C3 performed better at reducing the abundance of ARGs compared with C1 and C2.These results suggest that C3 is advantageous for effectively removing ARGs,thereby reducing the potential risk of ARGs spreading in the environment.According to the principal coordinate analysis,the structures of the bacterial communities changed significantly during AD and they were grouped according to the stages of AD in all control reactors.Network analysis suggested that Solibacillus,Enterococcus,Facklamia,Corynebacterium₁,and Acinetobacter may be potential hosts of ermB,sul1,and dfrA7.Redundancy analysis indicated that RDA1 and RDA2 accounted for 79.1% of the total variation in the ARGs.Among the selected variables,the bacterial communities?based on the abundances of phyla?explained the most variation,i.e.,46.7%,while environmental factors?SCOD,pH,NH4+,and VFAs?accounted for 30.8% of the total variations in the overall ARGs profiles during AD.This indicates that both the succession of the bacterial community and changes in environmental factors influenced the evolution of ARGs.