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Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria (blue), a cause of gastritis (stomach lining inflammation). These Gram- negative, rod- shaped, bacteria are commonly found in the mucosal lining of the stomach in middle-aged people. Their presence is also associated with the formation of ulcers in the duodenum (part of the small intestine). Magnification: x3100 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID MCCARTHY)