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 1.0–1.2 µm and a length of 3–5 µm. 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.