Decay of Viral Indicator T7 Bacteriophage After Repeated Exposures to Chlorine and Heat Treatments

Free chlorine and thermal treatment are used in water disinfection, sludge treatment, and food sterilization processes to inactivate pathogens. Previous research has shown that RNA viruses and bacteriophages can evolve to develop increased resistance to chlorination and thermal disinfection. However, DNA bacteriophages are also commonly used as indicators and surrogates for enteric viruses, but knowledge about their adaptation to disinfection processes is more limited. We repeatedly subjected T7 coliphage to small doses of free chlorine or to heat treatment at 60 degrees C for a long enough time to cause an inactivation of 90-99.9%, then cultivated the surviving phage and assessed the resistance to these treatments to understand if increased resistance emerged. After 10 repeated exposures to free chlorine, T7 coliphage did not develop increased resistance compared with the unexposed strain. However, T7 did develop an increased resistance to thermal treatment at 60 degrees C after four consecutive exposures. The heat-adapted population of T7 had a significantly lower decay rate than the original strain, but its breakpoint temperature (a measure of its thermal stability) did not change. Results from this study provide important implications for scientific and practitioner communities, specifically that not all bacteriophages are equal indicators, proxies, or surrogates for human pathogens in all treatment processes, and caution is warranted when choosing an appropriate indicator. Further research is needed to better understand the influence of virus genome type, mutation rate, and capsid reactivity on virus evolvability to chlorine and heat treatment processes.

Automated summary

This study found that T7 bacteriophage did not develop increased resistance after repeated exposures to free chlorine, but it did develop increased resistance to thermal treatment at 60 degrees C. This suggests that different bacteriophages have different resistance capabilities in different treatment processes, and caution is needed when choosing appropriate indicators or surrogates.