Free chlorine is used in water disinfection processes to inactivate pathogens by causing irreversible damage to proteins and nucleic acids. Previous research has shown that RNA viruses can evolve to develop increased resistance to chlorination and thermal disinfection. I repeatedly subjected a laboratory strain of T7 coliphage to doses of free chlorine or heat treatment at 60°C, to cause an inactivation of 90% – 99.9%, and then regrew the surviving phage and measured their resistance to these treatment methods, to understand if increased resistance emerged. After ten repeated exposures, T7 coliphage did not appear to have developed any increased resistance to free chlorine compared to the original stock. However, T7 developed an increased resistance to thermal disinfection at 60°C after only four exposures, as evidenced by the significantly lower decay rate coefficient for the heat-adapted strain compared to the original strain.
