Coloured transmission electron micrograph (TEM) of E. coli strains undergoing conjugation via a pilus (one strain has pili). Bacterial conjugation is the ability to transfer DNA between strains of bacteria (via a pilus). It allows a new mutation to spread through an existing population. It is believed that this process led to the spread of toxin synthesis from Shigella to E. coli (0157:H7). Escherichia 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 diarrhoea and nosocomial infections. The E. coli 0157:H7 strain is fatal to humans if contracted when contaminated meat is cooked inadequately. Magnification: x3,645 when shortest axis printed at 25 millimetres.