Bacillus /bəsil″əs/ , 1. a genus of aerobic, gram-positive, or facultatively anaerobic, spore-forming, rod-shaped microorganism of the family Bacillaceae, order Eubacteriales. The genus includes 34 species, 3 of which are pathogenic and the rest saprophytic soil forms; 25 species are considered medically important. Some species are nonpathogenic, but others cause a wide variety of diseases, ranging from anthrax (caused by B. anthracis) to tuberculosis. Many microorganisms formerly classified as Bacillus are now classified in other genera. See also acid-fast bacillus, Bacillaceae. 2. any rod-shaped bacteria.
