The occurrence and variations of extracellular antibiotic resistance genes in drinking water supply system: A potential risk to our health

At present, intracellular antibiotic resistance genes (iARGs) have been detected in every part of the drinking water supply system (DWSS). Still, the extracellular antibiotic resistance genes (eARGs) lack research in DWSS, which are essential in the spread of antibiotic resistance and pose a potential threat to human health. This study aims to investigate the pollution level and changing trends of eARGs in DWSS and to analyze the causes and influencing factors. Here, the raw water, the filtered water, the finished water from the drinking water treatment plant, the tap water from four distribution network sites, and the biofilm of different layers of the distribution system were sampled and analyzed from October 2019 to July 2021. We found that the drinking water treatment process before chlorination increased the total eARGs absolute abundance, up to 8.64 folds. In contrast, chlo-rination disinfection removed most eARGs (average removal rate of over 78.18%). The drinking water trans-portation increased the eARGs content. The absolute abundance of total eARGs (7.53 x 105-7.26 x 107 copies/L) in tap water was significantly higher than that of iARGs (2.63 x 105-3.26 x 106 copies/L) (p < 0.01, n = 13), indicating that the eARGs pollution was more severe than iARGs in drinking water distribution system. And the relative abundances of total eARGs and total iARGs in each biofilm sample were generally higher than those in water samples (2.51 folds and 2.20 folds, respectively). Hence the biofilm was an incubator of eARGs and iARGs. In addition, the DO, pH, and microorganisms were found as important driving forces for the change of eARGs. This study will be conducive to the risk evaluation and transmission control of antibiotic resistance.