Disinfection Kinetics of Virus Aggregates of Bacteriophage MS2

The aim of this study was to determine the difference in disinfection between aggregated and dispersed virus. Bacteriophage MS2 was chosen as a surrogate for enteric virus. The first part of this project consisted of characterizing the aggregation of MS2 and the second part examined differences in the disinfection of aggregated and dispersed virus solutions. It was shown that MS2 only aggregated at a pH below its isoelectric point when the solution ionic strength was below 50 mM. At neutral pH, high ionic strength and CaCl2 did not cause aggregation. High ionic strength showed inhibition of aggregation at low pH. Aggregation was also influenced by the choice of buffer. The effect of the solution chemistry on the extent of aggregation at pH 3 was the following: acetic acid (20 mM)>sodium chloride (20 mM)> phosphate buffer (5 mM PO42-, 10 mM NaCl). Disinfection of the virus was thus tested at a pH above and under the isoelectric point at low ionic strength (phosphate buffer and 20 mM NaCl) in order to have dispersed and aggregated MS2 virus, respectively. Ultraviolet light (UV) and peracetic acid (PAA) were the chosen disinfection techniques as they are generally regarded as pH independent. UV showed greater disinfection capacity for the aggregated system than for the mono- dispersed system. Both disinfection curves showed first order kinetics. This observed difference in disinfection rates may not be due to the aggregation state of the virus but to other mechanisms, including the effect of pH on direct and indirect oxidation (by ROS) of amino acids in the virus capsid proteins. This is supported by the fact that UV can penetrate within the virus unlike a chemical disinfectant which must be in physical contact with a virus component for a reaction to take place. In order to test the effect of direct contact by chemical disinfectants, an aggregated and a dispersed solution were spiked with PAA. A difference in the behavior of the disinfection curves in phosphate buffer was observed after an inactivation of 6 logs. The disinfection rate of the aggregated system slowed down (tailing) after some initial amount of disinfection, while the dispersed system was completely disinfected with one disinfection rate. This implies a solution with some large aggregates and many single viruses in the solution. In 20 mM NaCl the disinfection rate of the aggregated virus was much slower than for the dispersed system (kaggregated= 0.0224; kdispersed= 0.0747). Both inactivation curves showed first order kinetics. By applying Grant’s model (1995) an aggregation factor was calculated (b= 1.413). This indicates a very aggregated state with large aggregates.