hours (Moir, 2006; Setlow, 2003). The stability of spores, with accompanying ease of storage and transport, is also a major factor in their potential utility as a biological weapon.
The resilience of B. anthracis spores derives from their unique physical and structural characteristics. The spore cytoplasm or “core” is relatively dehydrated and contains high concentrations of certain low-molecular-weight solutes (Setlow, 2006). These conditions result in complete metabolic dormancy and extreme protein stability. The low core water content of the spore correlates with a high spore density, a property that is commonly exploited for spore purification. Centrifugation through density gradients of diatrizoate and meglumine is a widely used purification method (Tamir and Gilvarg, 1966; Nicholson and Setlow, 1990).
Surrounding the spore core are a membrane and a specialized peptidoglycan cell wall, the “cortex.” These structures play key roles in limiting movement of water and solutes into and out of the core, maintaining the dehydrated dormant state (Setlow, 2006). The membrane and cortex have not been cited as significant sites of mineral association nor found to play roles in direct spore interactions with external surfaces.
The outermost layers of the spore, the coat and exosporium, are the primary sites of interaction with the host and with other surfaces and are important factors in the resistance of spores to certain enzymatic and chemical treatments (Driks, 2009; Henriques and Moran, 2007; Setlow, 2006). These structures are composed predominantly of protein and glycoprotein. In some spore-forming bacteria, the spore coats have been shown to be sites of association of minerals (Johnstone et al., 1980; Mann et al., 1988; Stewart et al., 1980, 1981; Hirota et al., 2010). These outermost integuments can play major roles in determining spore adherence to surfaces (Bozue et al., 2007; Brahmbhatt et al., 2007) and may affect electrostatic properties and aggregation with other spores or particles, all of which will affect spore dispersal and infectivity. Efforts at spore aerosolization and dispersal have been pursued through modification of the spores’ surface structures and properties (Swartz, 2001).
The fact that B. anthracis can exist in the environment as a dormant, highly stable spore may have evolutionary significance. While a dormant environmental state may help explain observations that populations of B. anthracis have much less genetic variation than many other bacterial species (Van Ert et al., 2007a), other factors may also contribute, including the likely recent origin of this species and the possibility of limited means for horizontal gene transfer.
Anthrax is generally a disease of herbivores (e.g., cattle, sheep, horses), which acquire the infection by grazing on contaminated soils. Anthrax spores are highly resistant to environmental insults. These spores allow the bacterium