Helicobacter pylori. Coloured transmission electron micrograph of Helicobacter pylori bacteria (formerly called Campylobacter pyloridis), the cause of stomach ulcers. These Gram-negative bacilli bacteria are curved or spiral in shape, with flagellae for motility. Colonies occur on the mucous membrane of the stomach in people suffering from stomach ulcers (peptic ulcers). The species may also be linked to cancer of the stomach. Magnification: x23, 530 at 7x6cm size. x28,000 at 4x5 inch size.