Bacillus cereus is a Gram-positive, rod-shaped, facultatively anaerobic, motile, beta-hemolytic, spore forming bacterium commonly found in soil and food. The specific name, cereus, meaning "waxy" in Latin, refers to the appearance of colonies grown on blood agar. Some strains are harmful to humans and cause foodborne illness, while other strains can be beneficial as probiotics for animals.The bacteria is classically contracted from fried rice dishes that have been sitting at room temperature for hours.[3][4] B. cereus bacteria are facultative anaerobes, and like other members of the genus Bacillus, can produce protective endospores. Its virulence factors include cereolysin and phospholipase C.The Bacillus cereus group comprises seven closely related species: B. cereus sensu stricto (referred to herein as B. cereus), B. anthracis, B. thuringiensis, B. mycoides, B. pseudomycoides, and B. cytotoxicus

B. cereus competes with other microorganisms such as Salmonella and Campylobacter in the gut; its presence reduces the numbers of those microorganisms. In food animals such as chickens,rabbits and pigs,some harmless strains of B. cereus are used as a probiotic feed additive to reduce Salmonella in the animals' intestines and cecum. This improves the animals' growth, as well as food safety for humans who eat them. B. cereus can parasitize codling moth larvae.

B. cereus and other members of Bacillus are not easily killed by alcohol; they have been known to colonize distilled liquors and alcohol-soaked swabs and pads in numbers sufficient to cause infection.

Some strains of B. cereus produce cereins, bacteriocins active against different B. cereus strains or other Gram-positive bacteria.