Bacillus anthracis is a Gram-positive rod-shaped bacterium best known as the cause of anthrax. Unlike most members of the genus Bacillus, it is an obligate pathogen and relies on a vertebrate host for part of its life cycle. Each cell forms a single dormant spore surrounded by a robust coat that allows the organism to survive for decades in soil or animal products.
Explanation
This species grows as chains of large rectangular cells that can range from three to five micrometres long. When cultured on solid media the colonies often appear rough and have a slimy capsule. The bacterium carries two large plasmids, designated pXO1 and pXO2. Genes on pXO1 encode the lethal toxin and edema toxin that disrupt host cell signalling, while pXO2 carries genes for the poly‑D‑glutamic acid capsule that helps the cells evade phagocytosis. Infection usually begins when spores are taken up through a cut in the skin, inhaled or ingested. In animals and humans the spores germinate and vegetative cells multiply in lymphatic tissues, releasing toxins that cause tissue damage and systemic illness. Anthrax is primarily a zoonosis affecting grazing animals such as cattle and sheep, and humans become infected through contact with contaminated animals, hides or wool.
Clinical significance
There are several forms of anthrax: cutaneous anthrax produces a characteristic black eschar at the site of entry; inhalational anthrax results from inhaling spores and can progress rapidly to severe pneumonia and sepsis; gastrointestinal anthrax follows ingestion of contaminated meat. Early diagnosis and antibiotic treatment are critical. Because the spores are so resilient and can be dispersed as aerosols, B. anthracis has been used as a biological weapon. Vaccines for animals and at‑risk humans exist, and proper handling of animal products reduces the risk of exposure. Although most people will never encounter this organism, its ability to persist in the environment and cause severe disease makes it an important subject of public health and biodefense. Related Terms: Bacillus, Bacteria, Biohazard, Bacteremia, Broad-spectrum Antibiotics