Color enhanced transmission electron micrograph (TEM) of Escherichia coli, a rod shaped bacteria. Normally E. coli serves a useful function by suppressing the growth of harmful bacterial and synthesizing vitamins in the intestinal tract. A minority of E. coli strains can cause human illness. E. coli O157:H7 is a rare variety that produces potent toxins that cause severe damage to the intestine through diarrhea and internal bleeding. These toxins are closely related to the toxin produced by Shigella dysenteriae, the organism that causes bacterial dysentery.