Inactivation and tailing during UV254 disinfection of viruses: contributions of viral aggregation, light shielding within viral aggregates and recombination

UV disinfection of viruses frequently leads to tailing after an initial exponential decay. Aggregation, light shielding, recombination or resistant virus sub-populations were proposed as explanations; however, none of these options has conclusively been demonstrated. This study investigates how aggregation affects virus inactivation by UV254 in general, and the tailing phenomena in particular. Bacteriophage MS2 was aggregated by lowering the solution pH before UV254 disinfection. Aggregates were redispersed prior to enumeration to obtain the remaining fraction of individual infectious viruses. Results showed that initial inactivation kinetics were similar for viruses incorporated in aggregates (up to 1000 nm in radius) and dispersed viruses; however, aggregated viruses started to tail more readily than dispersed ones. Neither light shielding, nor the presence of resistant sub-populations could account for the tailing. Instead, tailing was consistent with recombination arising as a result of the simultaneous infection of the host by several impaired viruses. We argue that UV254 treatment of aggregates permanently fused a fraction of viruses, which increased the likelihood of multiple infection of a host cell and ultimately enabled the production of infective viruses via recombination.