E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria (red) taken from the small intestine of a child. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. However some strains, including this one, produce a toxin that causes severe diarrhoea and can be fatal, especially in the very young or elderly. Magnification: ×5,700 when printed at 10 centimetres wide. Cover credit: STEPHANIE SCHULLER / SCIENCE PHOTO LIBRARY.