Bacillus anthracis spores are agents used for biological warfare owing to high virulence, and extreme resistance to environmental stresses with negligible degradation in viability. There is an interest in understanding the persistence and subsequent health risks due to B. anthracis spores in the indoor environment during bioterror attacks, but neither the time-dependent behavior of B. anthracis spores on porous media nor the suitability between B. anthracis and its recommended experimental surrogate, B. thuringiensis spores, are well known. This work specifically focused on recovery of B. anthracis and B. thuringiensis spores from HVAC filters, a porous fomite, considering the effects of sampling time and moisture content using culture-based quantification. Effects of time significantly affected the recovery of Bacillus spores. Although there was an overall reduction in viability of Bacillus spores over time, the reduction in recovery of B. anthracis and B. thuringiensis varied remarkably. Clumping in B. thuringiensis spores and accelerated dehydration rates at higher moisture levels might have resulted in increased observed recovery of B. thuringiensis. Hence it is concluded that, B. thuringiensis spores is not an ideal experimental surrogate for B. anthracis in recovery studies. Inclusion of time and moisture emphasizes the importance of variation in recoveries and inferences between the surrogate and B. anthracis spores. The data generated in this study will be useful for persistence studies that measure reduction or decay but are dependent on recovery. Furthermore, this information will inform quantitative microbial risk assessment (QMRA) models that estimate the risk of exposure to B. anthracis spores in indoor environments.
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