E. coli bacterium. Coloured scanning electron micrograph (SEM) of Escherichia coli bacterium. 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. The hair-like tendrils are the flagella, which the bacterium uses as propellers to move around. Magnification: x 25, 000 when printed at 10 centimetres high.