Escherichia coli. Coloured transmission electron micrograph (TEM) of a colony of Escherichia coli, a gram-negative bacilli (rod-shaped) bacterium. Two bacteria (at upper left and far right) are dividing asexually by binary fission. While E. coli is a normal, usually harmless inhabitant of the human intestine, under certain conditions its numbers may increase, causing infection. In children, some strains cause gastroenteritis, while in adults it is responsible for traveller's diarrhoea" and most urinary tract infections, such as cystitis. E. coli is widely used in genetic studies and for genetic engineering. Magnification: x3000 at 35mm size."