Chlorine disinfection plays an important role in preventing and controlling waterborne disease outbreaks globally. Theoretically, it should help diminish or even eliminate antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in water. However, the findings about the enrichment of ARGs levels in the finished water after chlorination revealed by Liu et al made us realize that chlorine disinfection could enrich the ARGs in bacteria and promoted the transmission of ARB in water.
To explore the reasons for it, ARGs released from killed ARB, and culturable chlorine-injured bacteria produced in the chlorination process as the recipient, were investigated to determine their contribution to the horizontal transfer of ARGs during disinfection treatment. We discovered that Escherichia coli, Salmonella aberdeen, Pseudomonas aeruginosa and Enterococcus faecalis showed diverse resistance to sodium hypochlorite, and transferable RP4 could be released from killed sensitive donors consistently. Meanwhile, the survival of chlorine-tolerant injured bacteria with enhanced cell membrane permeabilisation and a strong oxidative stress-response demonstrated that the physiologically competent cells could be transferred by RP4 with an improved transformation frequency of up to 550 times compared with the corresponding untreated bacteria. Furthermore, water quality factors involving Chemical Oxygen Demand (CODMn), ammonium nitrogen and metal ions (Ca2+ and K+) could significantly promote above transformation frequency of released RP4 into injured E. faecalis.
Overall, our findings demonstrated that the chlorination process promoted the horizontal transfer of plasmids by natural transformation, which resulted in the exchange of ARGs across bacterial genera and the emergence of new ARB, as well as the transfer of chlorine-injured opportunistic pathogens from non-ARB to ARB. Considering that the transfer elements were quite resistant to degradation through disinfection, this situation poses a potential risk to public health.