 Escherichia coli ( / <unk> <unk> / ; also known as E. coli ) is a gram @-@ negative , facultatively anaerobic , rod @-@ shaped 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 serotypes can cause serious food poisoning in their hosts , and are occasionally responsible for product recalls due to food contamination . The harmless strains are part of the normal flora of the gut , and can benefit their hosts by producing vitamin K2 , and preventing colonization of the intestine with pathogenic bacteria . 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 . 
