Anthrax: Virulence and Vaccines

Anthrax is an infectious disease that has afflicted humans and their domestic livestock since ancient times. Although in many industrialized countries the disease is controlled by vaccination and good practices in rearing livestock, it remains a serious problem in many less developed regions of the world.

The causative organism, Bacillus anthracis, is a spore-forming, rod-shaped bacterium that inhabits the soil. Although the causal relation between the organism and anthrax has been known for more than a century, since the time of Koch and Pasteur, the specific factors responsible for the virulence of the organism have been identified and characterized during the last 30 years. The precise molecular basis for virulence has been elucidated only during the past decade.

Fully virulent strains of Bacillus anthracis possess two unique virulence factors: a poly-D-glutamic acid capsule that inhibits phagocytosis [1] and a tripartite toxin composed of protective antigen, edema factor, and lethal factor [2]. Capsules are produced by virulent strains of Bacillus anthracis growing in vivo and by cells grown on media containing serum or bicarbonate or both and incubated in a CO₂-enriched atmosphere.

The existence of an anthrax toxin was first demonstrated in 1955 in experiments that showed that injection of sterile plasma from infected guinea pigs resulted in local edema and death. Studies by American and British investigators during the ensuing decade showed that the toxin contained three separate components. The individual toxin components have no known biological effects when administered alone, but edema factor injected with protective antigen into the skin of rabbits or guinea pigs causes local edema, and protective …