Bacterial contamination, coloured scanning electron micrograph (SEM). Escherichia coli bacteria in a cell culture. This contamination has come from an unclean water source. 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: x6500 when printed at 10 centimetres across.