E. coli bacterium. Animated 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 structure is the flagellum, which the bacterium uses as a propeller to move around.