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Bacillus anthracis Gram stain morphology

image courtesy of textbookofbacteriology.net

Bacillus species are large motile, facultative anaerobic, gram positive bacilli with a central spore. The spore is quite resistant to extreme conditions and can survive in nature for prolonged periods of time.

Bacillus anthracis is non-motile and in Gram stain is often seen in chains. The virulent forms of B. anthracis is more likely to be surrounded by a capsule. The organism can be cultured as large colonies on blood agar plates within 24 hours, often resembling a “Medusa head” (irregular appearance to the colony with swirling projections).

The principal virulence factors of B. anthracis are the capsular polypeptide  polypeptide and the anthrax toxin. The capsule consist of poly- D- glutamine acid, which is thought to allow the organism to resit phagocytosis. Anthrax toxin consists of 3 proteins: protective antigen, edema factor, and lethal factor. Protective antigen is named for its ability to confer immunity in experimental situations. Edema factor and lethal factor bind to protective antigen to form edema toxin and lethal toxin. The bound proteins are transported across cell membranes and are released in the cytoplasm where they exert their effects. Once the spores enters the body, they are taken up by macrophages. Because of both lethal and edema factors, the spores survive killing, and subsequently germinate.