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Helicobacter pylori bacteria, coloured scanning electron micrograph (SEM). H. pylori is a Gram-negative helical bacterium found in the mucus lining of the stomach. It causes gastritis, and is also the most common cause of stomach ulcers. It may also be a cause or co-factor for gastric cancer as its presence increases the risk of developing stomach tumours. (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)