Coloured scanning electron micrograph (SEM) of an E. coli (normal strain) bacterium dividing. Escherichia coli (E. coli) is a Gram-negative, facultatively anaerobic, enteric, rod prokaryote. This bacterium was isolated from the human intestine and 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 diarrhea and nosocomial infections. The E. coli 0157:H7 strain is fatal to humans if contracted when contaminated meat is cooked inadequately. Magnification: x8,000 when shortest axis printed at 25 millimetres.