Inactivation of Bacillus endospores in envelopes by electron beam irradiation

The anthrax incidents in the United States in the fall of 2001 led to the use of electron beam (EB) processing to sanitize the mail for the U.S. Postal Service. This method of sanitization has prompted the need to further investigate the effect of EB irradiation on the destruction of Bacillus endospores. In this study, endospores of an anthrax surrogate, B. atrophaeus, were destroyed to demonstrate the efficacy of EB treatment of such biohazard spores. EB exposures were performed to determine (i) the inactivation of varying B. atrophaeus spore concentrations, (ii) a D10 value (dose required to reduce a population by 1 log10) for the B. atrophaeus spores, (iii) the effects of spore survival at the bottom of a standardized paper envelope stack, and (iv) the maximum temperature received by spores. A maximum temperature of 49.2 degrees C was reached at a lethal dose of approximately 40 kGy, which is a significantly lower temperature than that needed to kill spores by thermal effects alone. A D10 value of 1.53 kGy was determined for the species. A surface EB dose between 25 and 32 kGy produced the appropriate killing dose of EB between 11 and 16 kGy required to inactivate 8 log10 spores, when spore samples were placed at the bottom of a 5.5-cm stack of envelopes.