Effects of different composting methods on antibiotic-resistant bacteria, antibiotic resistance genes, and microbial diversity in dairy cattle manures

Composting is a common practice used for treating animal manures before they are used as organic fertil-izers for crop production. Whether composting can ef-fectively reduce microbial pathogens and antibiotic re-sistance genes remain poorly understood. In this study, we compared 3 different dairy manure composting methods-anaerobic fermentation (AF), static compost (SC), and organic fertilizer production (OFP)-for their effects on antibiotic-resistant bacteria, antibiotic resis-tance genes, and microbial community diversity in the treated manures. The 3 composting methods produced variable and distinct effects on antibiotic-resistant bac-teria, zoonotic bacteria, and resistance genes, some of which were decreased and others of which showed no significant changes during composting. Particularly, SC and OFP reduced chloramphenicol resistance gene fexA and opportunistic pathogen Vibrio fluvialis, whereas AF significantly reduced tetracycline resistance gene tetB and opportunistic pathogens Enterococcus faecium and Escherichia fergusonii. The compositions of microbial communities varied significantly during the composting processes, and there were significant differences be-tween the 3 composting methods. In all 3 composts, the dominant phyla were Firmicutes, Proteobacteria, Bacte-roidetes, and Actinobacteria. Interestingly, Firmicutes, Proteobacteria, and Bacteroidetes remained stable in the entire AF process, whereas they were dominated at the beginning, decreased at the early stage of com-posting, and rebounded at the later stage during SC and OFP. In general, SC and OFP produced a more profound effect than AF on microbial community diver-sities, pathogens, and dominant species. Additionally, Enterococcus aquimarinus was isolated from AF for the first time. Phylogenetic Investigation of Communi-ties by Reconstruction of Unobserved States function prediction analysis indicated that the genes related to membrane transport and amino acid metabolism were abundant in the 3 composts. The metabolism of amino acids, lipids, and carbohydrates increased as compost-ing progressed. The biosynthesis of antibiotics was en-hanced after fermentation in the 3 composting methods, and the increase in the SC was the most obvious. These results reveal dynamic changes in antibiotic-resistant bacteria, antibiotic resistance genes, microbial commu-nity composition, and function succession in different dairy manure composts and provide useful information for further optimization of composting practices.