Bacillus anthracis is the etiologic agent of anthrax — a common disease of livestock and, occasionally, of humans — and the only obligate pathogen within the genus Bacillus. B. anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.
It is one of few bacteria known to synthesize a protein capsule (poly-D-gamma-glutamic acid). Like Bordetella pertussis, it forms a calmodulin-dependent adenylate cyclase exotoxin known as (edema factor), along with lethal factor. It bears close genotypical and phenotypical resemblance to Bacillus cereus and Bacillus thuringiensis. All three species share cellular dimensions and morphology. All form oval spores located centrally in an unswollen sporangium. B. anthracis spores in particular are highly resilient, surviving extremes of temperature, low-nutrient environments, and harsh chemical treatment over decades or centuries.
The spore is a dehydrated cell with thick walls and additional layers that form inside the cell membrane. It can remain inactive for many years, but if it comes into a favorable environment, it begins to grow again. It is sometimes called an endospore, because it initially develops inside the rod-shaped form. Features such as the location within the rod, the size and shape of the endospore, and whether or not it causes the wall of the rod to bulge out are characteristic of particular species of Bacillus. Depending upon the species, the endospores are round, oval, or occasionally cylindrical. They are highly refractile and contain dipicolinic acid. Electron micrograph sections show that they have a thin outer spore coat, a thick spore cortex, and an inner spore membrane surrounding the spore contents. The spores resist heat, drying, and many disinfectants (including 95% ethanol).