Identification of an in vivo inhibitor of Bacillus anthracis spore germination

Germination of Bacillus anthracis spores into the vegetative form is an essential step in anthrax pathogenicity. This process can be triggered in vitro by the common germinants inosine and alanine. Kinetic analysis of B. anthracis spore germination revealed synergy and a sequential mechanism between inosine and alanine binding to their cognate receptors. Because inosine is a critical germinant in vitro, we screened inosine analogs for the ability to block in vitro germination of B. anthracis spores. Seven analogs efficiently blocked this process in vitro. This led to the identification of 6-thioguanosine, which also efficiently blocked spore germination in macrophages and prevented killing of these cells mediated by B. anthracis spores. 6-Thioguanosine shows potential as an anti-anthrax therapeutic agent. The 2001 anthrax bioterrorist attacks rekindled interest in the anthrax causative agent, Bacillus anthracis. The spores derived from B. anthracis survive exposure to extreme temperatures and harsh chemicals (1, 2) and represent an infectious agent with significant potential for biowarfare and bioterrorism. The severity of anthrax pathogenicity is dependent on the uptake route. Following inhalation, the most toxic route, B. anthracis spores are phagocytosed by alveolar macrophages in the lungs of infected hosts (3, 4). During the migration of infected macrophages to regional lymph nodes, B. anthracis spores germinate into the vegetative form (4), which secretes the anthrax toxins, the primary virulence factors of the bacterium (5–10). The transformation process from a dormant spore to a fully vegetative bacterium is a critical initial step in anthrax pathogenicity (3, 11). Therefore, drugs that prevent germination of phagocytosed spores are attractive candidates for controlling the anthrax disease. The transformation into vegetative bacteria is a two-step process in vitro. Following activation of spores by heavy metals, heat, or hydrostatic pressure, bacterium-specific germinants