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Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria on the surface of the human gut. It was formerly known as Campylobacter pyloridis. These Gram-negative bacilli (rod-shaped) bacteria are curved or spiral in shape. Hair-like flagellae (grey) are visible which help them to move. Colonies of H. pylori occur on the stomach mucous membrane in people who suffer from gastritis, and this bacteria has been linked to stomach ulcer formation. H. pylori may also be a co-factor for gastric cancer as its presence increases the risk of stomach tumours. Magnification: x16,000 at 6x6cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE)