Helicobacter pylori. Coloured transmission electron micrograph of two Helicobacter pylori bacteria (formerly called Campylobacter pyloridis). These Gram-negative bacilli bacteria are curved or spiral in shape, with flagellae for motility. Here, three hair-like flagellae are visible. Colonies of H. pylori occur on the stomach mucous membrane in people suffering gastritis, and this bacteria may be linked to stomach ulcer formation. H. pylori may also be a co-factor for gastric cancer; its presence increases the risk of stomach tumours. Magnification: x3750 at 6x4.5cm size.