E. coli bacteria. Coloured scanning electron micrograph (SEM) of Escherichia coli bacteria, a cause of food-borne illness. Certain strains of these Gram-negative intestinal bacteria (enterobacteria) can cause bloody diarrhoea and kidney failure, particularly in the elderly and the very young. The pathogenic strains are found in the intestines of some cattle, and can spread to humans in contaminated beef and milk. Transmission can be prevented by thoroughly cooking meat and pasteurising milk. Magnification: x5000 at 6x7cm size.