Coloured scanning electron micrograph (SEM) of Photocomposite, E. coli on the surface of human skin. Escherichia coli, Gram-negative, facultatively anaerobic, rod prokaryote. E. coli is part of human and animal microbiota. It can cause urinary tract infections, traveller's diarrhoea and nosocomial infections. Shown in this image is human skin (epidermis) with a bacteria density of 100 bacteria per square millimetre. Magnification: bacteria x200; skin x200 when shortest axis printed at 25 millimetres.