E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria. These bacteria are a normal part of the intestinal flora in humans and other animals, where they aid digestion. However, some strains, for instance E. coli O157, can produce a toxin that leads to severe illness, or even death. Normal strains can also produce infections in weakened or immunosuppressed people. Magnification: x1500 when printed 10 centimetres wide.