Inactivation of phage phiX174 by UV and free chlorine: Structure impairment and function loss.

Disinfection is widely applied in water and wastewater treatment to inactivate viruses. However, the inactivation mechanism associated with viral structural alteration during disinfection is still not clear. In this work, inactivation of bacteriophage phiX174 by ultraviolet radiation (UV) and free chlorine (FC), two most commonly used disinfection processes, was studied at the molecular level to investigate the relationship between phiX174 genome impairment and virus inactivation, and the correlation between protein impairment and function loss. Double-layer agar technique, quantitative real-time polymerase chain reaction (qPCR), real-time reverse transcription-polymerase chain reaction (RT-qPCR), and liquid chromatography-tandem mass spectrometry techniques (LC-MS/MS), together with structure impairment and function experiments were implemented to quantitatively analyze the inactivation and damage to genome and proteins of phiX174. Results showed that UV and FC could effectively inactivate phiX174 at the practical doses (UV dose of 30 mJ/cm, and FC of 1-3 mg/L) used in water treatment plants, accompanied with the damage to viral genome and proteins. Specifically, a UV irradiation dose of 9.6 mJ/cm, and FC at an initial concentration of 1 mg/L at 4 min could lead to a 4-log inactivation. Nevertheless, the combination of these two methods at selected doses played no significant synergistic disinfection effect. During UV disinfection, the proportion of phiX174 with damaged genome was similar with that of the inactivated phiX174. In addition, UV and FC could disrupt proteins of phiX174 such as H protein, thereby hindering the physiological function associated with these proteins. With these findings, it is suggested that UV and FC disinfection could hinder the injection of the viral genome into host cells, thus resulting in the inactivation of phiX174. This work provides a comprehensive study of the inactivation mechanism of phiX174, which can enhance the applicability of UV and FC in water treatment plants, and facilitate the design and optimization of disinfection technologies for virus control in drinking water and wastewater worldwide to ensure the biosafety.