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Bacterial biotyping. Female researcher holding a microtitre plate as she does biochemical typing (biotyping) on E. coli and Proteus sp. bacteria. Cultured bacteria have been placed into a variety of differently coloured growth media infused with chemicals on the plate. The reaction of the bacte- ria to these growth media allows the different bacterial strains to be identified. E. coli and Proteus bacteria can cause disease in humans. (KEYSTONE/SCIENCE PHOTO LIBRARY/GEOFF TOMPKINSON)