Escherichia coli, commonly shortened to E. coli, is a Gram-negative bacterium that lives in the intestinal tract of humans and other warm-blooded animals. There are many sub-types, or strains of the organism. One strain is designated as O157:H7, based on two antigens that are present on the surface of the bacterium and of the locomotive appendage called the flagella.

In contrast to many of the other strains, E. coli O157:H7 is not a normal resident of the humans intestinal tract. When present in the intestinal tract, via the ingestion of contaminated food or water, O157:H7 causes a severe, even life-threatening malady known as hemorrhagic colitis.

E. coli O157:H7 is a strain of enterohemorrhagic E. coli that was initially isolated in Argentina in 1977. The strain is thought to have arisen from a genetic recombination between another E...