UV or GAMMA Radiation for Inactivation of Bacillus Spores in Aqueous Suspension and on Surfaces

Bacillus anthracis spores represent an important bioterrorism agent that can be dispersed in air or water. The most serious threat to human health by B. anthracis spores is via the inhalation pathway. Therefore, transmission of the spores in dry form on or in parcels presents an important risk. It may also be possible to conduct a bioterrorist attack with these organisms through public water supplies. Although this method of attack will result in substantial public exposure through direct ingestion, the inhalation pathway may also be important for people in close proximity to water sprays (e.g., showers) as a result of spore aerolosolization. Existing decontamination practices based on these spores have focused on chemical disinfectants; however, the basic characteristics of radiation-based disinfectants suggest potential advantages in their application for control of Bacillus spores, including minimal disinfection by-product formation, and more generally the ability to accomplish microbial inactivation with little or no (other) change to the affected medium.Experiments were conducted to examine the effectiveness of UV254 radiation and γ radiation for inactivation of Bacillus spores in aqueous suspension and on dried surfaces. Spores of Bacillus cereus were used for most experiments because of their similarity to B. anthracis. A limited number of experiments were also conducted using B. anthracis Sterne spores.UV254 dose-response relationships for both spore types in aqueous suspension were characterized by a pronounced shoulder, followed by a region of nearly first-order inactivation, followed by tailing. For the conditions of culture and assay used in these experiments, B. anthracis Sterne spores were observed to be slightly more resistant to UV254 than the spores of B. cereus. Both spore types were more sensitive to UV254 radiation in aqueous suspension than B. subtilis spores, which are commonly used to characterize the performance of UV disinfection systems for water. Therefore, it may be reasonable to expect that the extent of B. anthracis spore inactivation accomplished by a UV system for water disinfection may be at least as great as that of B. subtilis spores. Dried spores on surfaces were observed to be more resistant to UV254 than the same spores in aqueous suspension; it is likely that the increased resistance to UV of the dried spores was attributable to surface characteristics (porosity and texture) of the solid materials.γ radiation was shown to accomplish similar rates of inactivation for spores in aqueous suspension and for dried spores on surfaces. Dissolved oxygen concentration had a nearly negligible influence on the γ dose-response behavior of Bacillus spores. The source of γ radiation used on these experiments (60Co) is a reasonable surrogate for several other sources of ionizing radiation, including 137Cs and electron beams.Collectively, these results suggest that the application of UV or ionizing radiation may hold promise for decontamination following bioterrorism events.