Coloured scanning electron micrograph (SEM) of Photocomposite, E. coli on the surface of human skin. Escherichia coli is a Gram-negative, facultatively anaerobic, enteric, rod prokaryote. This bacterium is normally a part of the human and animal microbiota. Most E. coli strains are harmless, but some strains can cause serious problems such as: food poisoning, urinary tract infections, traveller's diarrhoea and nosocomial infections. The E. coli 0157:H7 strain is fatal to humans if contracted when contaminated meat is cooked inadequately. Magnification: bacteria x200; skin x200 when shortest axis printed at 25 millimetres.