Gram-positive bacteria which are ubiquitous in nature (soil, water, and airborne dust). Some species are natural flora in the human intestines. A characteristic of this bacterium is its ability to produce endospores when environmental conditions are stressful. One of about 6 genera of endospore-forming bacteria, the best known is Clostridium. Hundreds of species have been named in Bacillus, but only about 40 are now recognized. Although most species of Bacillus are harmless saprophytes, two species are considered medically significant: B. anthracis and B. cereus. Most species move with flagella attached around the periphery of the cell. There are many ecophysiological groups including extremophiles (acidophiles, alkaliphiles, halophiles, psychrophiles, thermophiles) as well as denitrifiers, nitrogen fixers, producers of antiobiotics, pathogens of various kinds of organisms. Includes causative agent of anthrax.