E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria. These bacilli (rod-shaped bacteria) are part of the normal content of the digestive systems of humans and other animals. However, under certain conditions they can cause gastroenteritis and urinary tract infections. Some strains also cause food poisoning. Magnification: x3,900 when printed at 10 centimetres wide.