Chlorine-resistant bacteria in drinking water system: Detection characteristics, identification, and chlorine...

The long-term and widespread use of chlorine disinfectants have led to the emergence of chlorine-resistant bacteria (CRB), which has been frequently detected in drinking water system. Some CRB are pathogenic and may pose risks to public health. This study investigated the occurrence of CRB in the drinking water system of a megacity in eastern China and explored the mechanisms underlying chlorine resistance in typical CRB. The proportions of CRB among total culturable bacteria in source water, tap water, and potable water were 0.05-16.8 %, 39.9-100 %, and 0-45.5 %, respectively. Mycobacterium, Methylobacterium, and Bacillus were the most prevalent shared CRB in the three water samples. Mycobacterium mucogenicum TW-3 was identified as a typical CRB based on its high detection frequency (100 %), high concentration (up to 26 CFU/mL), and notable resistance to chlorine disinfectant. Its resistance to reactive chlorine was attributed to the consumption of disinfectants by the extracellular polymeric substances, repair of membrane damage, and upregulation of antioxidant enzymes. Furthermore, TW-3 activated a thioredoxin-centered reducing system to mitigate chlorine dioxide-induced oxidative stress. These findings present important insights into the mechanisms underlying chlorine resistance in CRB and provide theoretical support for improving microbial safety control in the drinking water system.