Bacillus anthracis, the causative agent of anthrax, is a gram-positive, facultatively aerobic, spore-forming rod. It is unique in that it is the only bacterium with a capsule composed of protein. This unique feature, which is required for full virulence, helps the bacterium to evade the immune system by preventing phagocytosis by macrophages. Additionally, this bacterium also produces and releases an exotoxin composed of three proteins: edema factor, protective antigen, and lethal factor. Each protein by itself is nontoxic, but together they produce the systemic effects of anthrax.

B. anthracis is commonly found in herbivores such as cows and sheep. Long-surviving B. anthracis spores may be found in the soil near pastures contaminated by infected animals or in animal by-products such as wool or hides. Humans are susceptible to B. anthracis and may develop anthrax after coming in contact with infected animals or their by-products.

Anthrax manifests itself in three forms: cutaneous anthrax, respiratory anthrax, or gastrointestinal anthrax. Cutaneous anthrax is the most common form of the disease. The exotoxin causes a painless round black lesion on the skin 2 to 5 days after exposure, the result of localized tissue necrosis. Penicillin treatment is effective in this manifestation of the disease. Without penicillin therapy, the bacteria will continue to proliferate and enter the bloodstream, possibly causing death. Gastrointestinal anthrax, a rare form of the disease, is caused by ingestion of spores from improperly cooked, contaminated meat. B. anthracis is able to mature and replicate within the intestine, causing a necrotic lesion. Symptoms of gastrointestinal anthrax include vomiting, abdominal pain, and bloody diarrhea. Mortality rates are high (25% to 60%) if untreated. Inhalational anthrax, also known as Woolsorter's disease, occurs when the spores are inhaled. The bacteria mature and replicate in the lungs, where the exotoxin is released. Symptoms, which include fever, cough, malaise, fatigue, and body aches, generally develop within 7 days of exposure. Respiratory distress and death quickly follow in 95% to 100% of untreated cases.

Although cases of anthrax are rare in the United States, the causative agent has attracted attention lately due to its potential use in bioterrorism and biological warfare. Anthrax spores have been aerosolized for use in biological weapons. A 1979 outbreak of anthrax in Sverdlosk, Russia, is now attributed to an accidental release of anthrax spores from a bioweapons facility operating under the old Soviet bioweapons program.

See also Bacillus anthracis—Series 2.

Figure 1: B. anthracis colony morphology on blood agar. Colonies are gray and irregularly shaped with a rough, uneven surface. There may be comma-shaped extensions at the edge resembling a "Medusa head." Photo credit: James Feeley, Centers for Disease Control and Prevention.

Figure 2: Gram stain of B. anthracis.

Figure 3: Carbol fuchsin stain of B. anthracis taken from heart blood.

Figure 4: Fuchsin-methylene blue spore stain of B. anthracis from an agar culture demonstrating spores.