Which bacterium is the causative agent of anthrax?

The bacterium that causes anthrax is Bacillus anthracis. It is a Gram-positive, rod-shaped, spore-forming bacterium that infects people and other mammals.

B. anthracis normally persists as highly resistant spores in soil. Grazing animals can ingest or inhale the spores, while humans may become infected through contaminated animal products, inhalation, ingestion, or skin wounds. Once inside a suitable host, the spores germinate into growing bacterial cells. The disease has cutaneous, inhalational, gastrointestinal, and injection-related forms.

The bacterium’s virulence depends heavily on two plasmids. One carries genes for a protective capsule that helps the cells evade immune defenses; the other encodes the three-part anthrax toxin system: protective antigen, edema factor, and lethal factor. These components contribute to swelling, immune disruption, tissue injury, and severe disease.

Robert Koch demonstrated B. anthracis as a cause of disease in 1876, a landmark event in the development of germ theory. Bacillus cereus is a close relative, and a specialized B. cereus lineage can also cause anthrax-like disease, but B. anthracis remains the canonical quiz answer.