Metabolic response of bacterial community to sodium hypochlorite and ammonia nitrogen affected the antibiotic resistance genes in pipelines biofilm

The biofilm is important for the antibiotic resistance genes (ARGs) propagation in drinking water pipelines. This study investigated the influence of chlorine disinfection and ammonia nitrogen on the ARGs in pipelines biofilm using metagenomic and metabolomics analysis. Chlorine disinfection reduced the relative abundance of unclassified_c_Actinobacteria, Acidimicrobium, and Candidatus_Pelagibacter to 394-430 TPM, 114-123 TPM, and 4954 TPM, respectively. Correspondingly, the ARGs Saur_rpoC_DAP, macB, and mfd was reduced to 8-12 TPM, 81-92 TPM and 30-35 TPM, respectively. The results of metabolomics suggested that chlorine disinfection suppressed the pathways of ABC transporters, fatty acid biosynthesis, biosynthesis of unsaturated fatty acids, and biosynthesis of amino acids. These pathways were related to the cell membrane integrality and extracellular polymeric substances (EPS) secretion. Chlorine disinfection induced the decrease of EPS-related genes, resulting in the lower relative abundance of bacterial community and their antibiotic resistance. However, added approximately 0.5 mg/L NH3-N induced up-regulation of these metabolic pathways. In addition, NH3-N addition increased the relative abundance of enzymes related to inorganic and organic nitrogen metabolic pathway significantly, such as ammonia monooxygenase, glutamine synthetase, and glutamate synthase. Due to the EPS protection and nitrogen metabolism, the relative abundance of the main bacterial genera and the related ARGs increased to the level equal to that in pipelines biofilm with no disinfection. Therefore, NH3-N reduced the ARGs removal efficiency of chlorine disinfection. It is necessary to take measures to improve the removal rate of NH3-N and ARGs for preventing their risks in drinking water.