E. coli bacteria. Coloured scanning electron micrograph of the rod-shaped, Gram-negative bacteria, Escherichia coli, 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. 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 causes traveller's diarrhoea" and 80% of all urinary tract infections. It is also the organism most used in genetic engineering studies. Magnification: x2400 at 6x7cm size."