E. coli bacteria. Coloured transmission electron micrograph (TEM) of a section through Escherichia coli bacteria. E. coli are rod shaped. The bacterium at lower centre appears round because of the angle of the section. These Gram- negative bacteria are enterobacters, normally harmless inhabitants of human and animal intestines. However, under certain conditions they can cause gastroenteritis and urinary tract infections. Some strains can cause food poisoning. Magnification unknown.