E.coli bacteria. Coloured scanning electron micrograph of the rod-shaped, Gram-negative bacteria, Escherichia coli, commonly known as E. coli. These bacteria are normal inhabitants of the human intestine (also animal intestines) and are usually harmless. Under certain conditions E. coli may 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 studies. Magnification: x3,000 at 6x7cm, x1,500 at 35mm size. x10,000 at 8x10""