Escherichia coli ( ) (E. coli), is a bacterium that is commonly found in the lower intestine of warm-blooded animals. Most E. coli strains are harmless, but some, such as serotype O157:H7, can cause serious food poisoning in humans, and are occasionally responsible for costly product recalls. PMID 15842119. The harmless strains are part of the normal flora of the gut, and can benefit their hosts by producing vitamin K 2 , Bentley, R, Meganathan, R., Biosynthesis of Vitamin K (menaquinone) in Bacteria, Bacteriological Reviews, 1982, 46(3):241-280. Review. or by preventing the establishment of pathogenic bacteria within the intestine. Hudault, S.; J. Guignot and A.L. Servin (July 2001). "Escherichia coli strains colonizing the gastrointestinal tract protect germfree mice against Salmonella typhimurium infection." Gut 49:47-55. PMID 11413110. Reid, G.; J. Howard and B.S. Gan (September 2001). "Can bacterial interference prevent infection?" Trends in Microbiology 9(9):424-428. PMID 11553454.

E. coli are not always confined to the intestine, and their ability to survive for brief periods outside the body makes them an ideal indicator organism to test environmental samples for fecal contamination. The bacteria can also be grown easily and its genetics are comparatively simple and easily-manipulated, making it one of the best-studied prokaryotic model organisms, and an important species in biotechnology. E. coli was discovered by German pediatrician and bacteriologist Theodor Escherich in 1885, and is now classified as part of the Enterobacteriaceae family of gamma-proteobacteria.