Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm- blooded organisms (endotherms). Most E. coli strains are harmless, but some can cause serious food poisoning in their hosts, and are occasionally responsible for food contamination incidents that prompt product recalls. The harmless strains are part of the normal microbiota of the gut, and can benefit their hosts by producing vitamin K2 (which helps blood to clot) and preventing colonisation of the intestine with pathogenic bacteria, having a mutualistic relationship. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for 3 days, but its numbers decline slowly afterwards. E. coli is a chemoheterotroph whose chemically defined medium must include a source of carbon and energy.