Helicobacter pylori. Coloured transmission electron micrograph of Helicobacter pylori bacteria (formerly called Campylobacter pyloridis). These Gram-negative bacilli bacteria are curved or spiral in shape, with flagellae for motility. Colonies of H. pylori occur on the stomach mucous membrane in people suffering from 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: x21,700 at 6x7cm size.