DNA phosphorothioate modification facilitates the dissemination of mcr-1 and blaNDM-1 in drinking water supply systems

The mechanism driving the dissemination of antibiotic resistance genes (ARGs) in drinking water supply systems (DWSSs) with multiple barriers remains poorly understood despite several recent efforts. Phosphorothioate (PT) modifications, governed by dndABCDE genes, occur naturally in various bacteria and involve the incorporation of sulfur into the DNA backbone. PT is regarded as a mild antioxidant in vivo and is known to provide protection against bacterial genomes. We combined quantitative polymerase chain reaction, metagenomic, and network analyses for the water treatment process and laboratory-scale experiments for chlorine treatment using model strains to determine if DNA PT modification occurred in DWSS and facilitated the dissemination of mobilized colistin resistance-1 (mcr-1) and New Delhi metallo-β-lactamase-1 (blaNDM₋₁) in DWSS. Our results indicated that the relative abundance of dndB increased in the effluent, compared with the influent, in the water treatment plants. Presence of dndB copies had a positive correlation with the concentration of chloramine disinfectant. Network analysis revealed Bdellovibrio as a potential host for MCR genes, NDM genes, and dndB in the DWSS. E. coli DH10B (Wild-type with the dndABCDE gene cluster and ΔdndB) model strains were used to investigate resistance to chlorine treatment at the concentration range of 0.5–3 mg/L. The resistance of the wild-type strain increased with increasing concentration of chlorine. DNA PT modification protected MCR- and NDM-carrying bacteria from chloramine disinfection during the water treatment process. The higher relative abundance of ARGs in the effluent of the water treatment plants may be due to the resistance of DNA PT modification to chloramine disinfection, thereby causing the enrichment of genera carrying MCR, NDM, and dndB. This study provides a new understanding on the mechanism of ARG dissemination in DWSS, which will help to improve the performance of drinking water treatment to control the risk associated with antibiotic-resistant bacteria.