A comparative study of synergistic effects in sequential inactivation of Cryptosporidium parvum oocysts and Bacillus subtilis spores with chemical disinfectants

Even though scientists have been able to develop viable disinfection strategies for the control of Cryptosporidium parvum oocysts in drinking water treatment, very limited work has been done on identifying and characterizing the interactions among chemical disinfectants and oocyst components involved in parasite inactivation. Furthermore, no definitive mechanistic explanation has been provided for the occurrence/absence of synergistic effects observed in the inactivation of this parasite with sequential combinations of chemical disinfectants. The objective of this study was to investigate potential interactions among chemical disinfectants and oocyst constituents that could be responsible for the inactivation of C. parvum oocysts during single-step and sequential disinfection. Since previous studies had suggested the possibility that chemical disinfectants could be reacting with common sites within the oocyst wall, this study focused on characterizing the disinfectant demand by C. parvum oocysts as well as its potential correlation with the occurrence---or absence---of synergistic effects in the sequential chemical inactivation of this pathogen. Because of remarked discrepancies in inactivation kinetics and the different structural and biochemical compositions of C. parvum oocysts and Bacillus subtilis spores, this study put special emphasis on utilizing B. subtilis spores as a second, well-characterized microorganism, in order to develop explanations about the mechanism of inactivation of the C. parvum oocysts that were consistent to the inactivation process of other microorganisms of potential interest. The chemical disinfectants of interest for this study were ozone, chlorine dioxide, free chlorine and monochloramine, and the combinations of interest were ozone (or chlorine dioxide) followed by either free or combined chlorine. It is believed that understanding the process of oocyst inactivation by chemical disinfectants might allow the elucidation of new, more effective strategies to inactivate not only C. parvum oocysts, but also other emerging waterborne pathogens.