BIOLOGY MEDIA / SCIENCE PHOTO LIBRARY BIOLOGY MEDIA / SCIENCE PHOTO LIBRARY
E. coli. Coloured Transmission Electron Micrograph (TEM) of a slice through a rod-shaped Escherichia coli bacterium. The multi-layered cell wall, char- acteristic of Gram-negative bacteria, shows up as a double line around the cell. The dark area in the bacterium is the nucleoid, a gel-like body containing the DNA. E. coli is a normally harmless inhabitant of the human gut, where it aids digest- ion and helps to make vitamin K. Some strains, however, are pathogenic, causing a type of gastro- enteritis commonly known as traveller's diarrhoea, or "Delhi belly". E. coli is used to test for sewage contamination in water and to clone genes in genetic engineering. Magnification unknown.
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