Coloured transmission electron micrograph (TEM) of Escherichia coli O157 bacteria. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. The O157 strain produces a toxin that can cause bloody diarrhoea and kidney failure. It can be fatal, especially in the very young or elderly. E. coli O157 is found in the intestines of some cattle, and can spread to contaminate beef and milk. Its transmission can be prevented by thoroughly cooking meat and through the pasteurisation of milk. Magnification: x20,800 when printed at 10 centimetres wide.