Escherichia coli, also known as E. coli is a gram-negative, facultatively 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 serotypes can cause severe 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, having a symbiotic relationship.

E. coli is expelled into the environment within fecal matter. The bacterium grows massively in a fresh fecal matter under aerobic conditions for three days, but its numbers decline slowly afterward.E. coli and other facultative anaerobes constitute about 0.1 % of gut flora, and fecal-oral transmission is the major route through which pathogenic strains of the bacterium cause disease.

Cells are able to survive outside the body for a limited amount of time, which makes them potential indicator organisms to test environmental samples for fecal contamination. A growing body of research, though, has examined environmentally persistent E. coli, which can survive for extended periods outside of a host.

The bacterium can be grown and cultured easily and inexpensively in a laboratory setting and has been intensively investigated for over 60 years.

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