E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria (blue) 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: x11,000 when printed at 10 centimetres wide.