E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria. E. coli are Gram-negative rod-shaped bacteria that are part of the normal flora of the human gut. Some strains produce a toxin that causes severe diarrhoea and can be fatal, especially in the very young or elderly. Magnification: x15MB0MB00 when printed at 10 centimetres wide.