Escherichia coli (E. coli), is one of many species of bacteria living in the lower intestines of mammals, known as gut flora.

When located in the large intestine, it assists with waste processing, vitamin K production, and food absorption. Discovered in 1885 by Theodor Escherich, a German pediatrician and bacteriologist, E. coli are abundant: the number of individual E. coli bacteria in the feces that a human defecates in one day averages between 100 billion and 10 trillion. However, the bacteria are not confined to this environment, and specimens have also been located, for example, on the edge of hot springs. The E. coli strain O157:H7 is one of hundreds of strains of the bacterium that causes illness in humans.

As with all Gram-negative organisms, E. coli are unable to sporulate. Thus, treatments which kill all active bacteria, such as pasteurization or simple boiling, are effective for their eradication, without requiring the more rigorous sterilization which also deactivates spores.

As a result of their adaptation to mammalian intestines, E. coli grow best in vivo or at the higher temperatures characteristic of such an environment, rather than the cooler temperatures found in soil and other environments.

E. coli can generally cause several intestinal and extra-intestinal infections such as urinary tract infections, meningitis, peritonitis, mastitis, septicemia and Gram-negative pneumonia. The enteric E. coli are divided on the basis of virulence properties into enterotoxigenic (ETEC, causative agent of diarrhea in humans, pigs, sheep, goats, cattle, dogs, and horses), enteropathogenic (EPEC, causative agent of diarrhea in humans, rabbits, dogs, cats and horses); enteroinvasive (EIEC, found only in humans), verotoxigenic (VTEC, found in pigs, cattle, dogs and cats); enterohaemorrhagic (EHEC, found in humans, cattle, and goats, attacking porcine strains that colonize the gut in a manner similar to human EPEC strains) and enteroaggregative E. coli (EAggEC, found only in humans).