Medical Definition of Bacillus anthracis
Bacillus anthracis: The bacterium that causes anthrax. Anthrax differs from most bacteria in that they exist in an inactive (dormant) state called spores. The spores are found in soil, animal carcasses and feces (including sheep, goats, cattle, bison, horses, and deer), and animal products (e.g., hides and wool). Some animals (cats, dogs, rats, and swine) are very resistant to anthrax. Remarkably, anthrax spores can remain dormant in soil for many years, perhaps decades. Likened somewhat to eggs that have the ability to hatch, spores can transform (germinate) into active bacteria under appropriate conditions.
The spores themselves do not cause significant damage to tissue. Once in the body, the spores germinate to form the virulent(disease-causing) bacteria. Thus, the spores can lead to disease by:
- entering broken skin and germinating there to cause cutaneous anthrax;
- being inhaled and germinating in the lungs to cause inhalation anthrax; or
- being eaten and germinating in the gastrointestinal tract to cause gastrointestinal anthrax.
Bacillus anthracis was discovered in 1850. Notably, it was actually the first bacterium to be shown to cause a disease. In fact, it was the great German physician, Robert Koch, who discovered this. He grew the anthrax bacteria in culture plates, injected them into animals, and thereby demonstrated that the bacteria produced the disease.
Then, the famous French scientist, Louis Pasteur (known for pasteurizing milk), used anthrax bacteria that he damaged to develop a vaccine for anthrax. His idea was that the damaged bacteria would not cause the disease, but would still protect (produce immunity) against anthrax. Indeed, he showed that this vaccine protected animals from getting the disease when they were subsequently injected with healthy, virulent (disease-causing) anthrax bacteria.
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