Anthrax: Encyclopedia II - Anthrax - Description of the bacterium

Anthrax - Description of the bacterium

Bacillus anthracis is a rod-shaped Gram-positive bacterium of size about 1 by 6 micrometres. It was the first bacterium ever to be shown to cause disease, by Robert Koch in 1877. The bacteria normally rest in spore form in the soil, and can survive for decades in this state. Once taken in by an herbivore, the bacteria start multiplying inside the animal and eventually kill it, then continue to reproduce in the carcass. Once they run out of nutrients there, they revert back to the dormant spore state.

The infection of herbivores (and humans) proceeds as follows: the spore is located and engulfed by scavenger cells of the immune system specialized to deal with invaders. Inside the scavenger cell, the spore turns into a bacillus, multiplies, and eventually bursts the cell, releasing bacilli into the bloodstream. There they release a protein toxin which has macrophages as its principal target. The toxin has two components: edema factor and lethal factor. Edema factor inactivates macrophages such that they cannot phagocytose bacteria.

Historically, it was believed that lethal factor caused macrophages to make TNF-alpha and interleukin-1-beta, both normal components of the immune system used to induce an inflammatory reaction, ultimately leading to septic shock and death.

However, recent evidence indicates that anthrax also targets endothelial cells, causing vascular leakage (similar to hemorrhagic bleeding), and ultimately hypovalemic shock, not septic shock.

The virulence of a strain of anthrax is dependent on multiple factors, primarily which include its poly-D-glutamic acid capsule which protects the bacterium from phagocytocis by host macrophages, and its toxins, Edema Toxin and Lethal Toxin.

Adapted from the Wikipedia article "Description of the bacterium", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki