Evolution of Bacillus anthracis, Causative Agent of Anthrax

Bacillus anthracis is the causative agent of anthrax, primarily a disease of herbivores but also a serious threat to humans when introduced into their environment by natural or nefarious means. The anthrax toxin complex is separated into three protein components called edema factor (EF), lethal factor (LF), and protective antigen (PA). It is clear that the classical anthrax-associated strains are monophyletic and that their global distribution is due to a clonal expansion. Important anthrax virulence factors are likely to have evolved before this phylogenetic bifurcation and are thus found in non-B. anthracis strains, some of which are even associated with severe pathological conditions. B. cereus sensu lato contains numerous pathogenic types whose attributes are controlled or greatly modified by virulence genes on large plasmids. The monophyletic population structure of B. anthracis sensu stricto contains at least three major and multiple minor clonal genetic divisions, estimated by several different molecular genetic methods that largely agree. Full virulence of B. anthracis and the manifestation of anthrax requires both capsule production, pXO2, and the tripartite toxin on pXO1. The pleiotropic effects of the plcR inactivation result in numerous phenotypic deficiencies that would impair strains from existing opportunistic pathogens in the environment.