Coloured scanning electron micrograph (SEM) of the rod-shaped, Gram-negative bacteria, Escherichia coli, commonly known as E. coli. These bacteria are normal inhabitants of the human intestine (they also occur in the intestine of animals) and are usually harmless as pathogens. Under certain conditions, however, E. coli might increase in number and cause infection. Serotypes of E. coli are responsible for gastro-enteritis in children, particularly in tropical countries. In adults it is the cause of traveller's diarrhoea"; and of 80% of all urinary tract infections. It is also the organism most used in genetic engineering studies. Magnification: x9600 at 6x6cm size."