Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria. The tubes connecting the bacteria are pili, which are used to transfer genetic material between bacteria in a process known as conjunction. 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: x27,800 when printed at 10 centimetres wide.