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 gastroenteritis in children, particularly in tropical countries. In adults it is the cause of "traveller's diarrhoea"