Anthrax toxin is a three-protein exotoxin secreted by virulent strains of the bacterium, Bacillus anthracis, the causative agent of anthrax. Anthrax toxin is composed of a cell-binding protein, known as protective antigen (PA), and two enzyme components, called edema factor (EF) and lethal factor (LF). Anthrax is caused by B. anthracis, a spore-forming, Gram positive, rod-shaped bacterium. The lethality of the disease is caused by the bacterium's two principal virulence factors: the polyglutamic acid capsule, which is anti-phagocytic, and the tripartite protein toxin, called anthrax toxin.