Helicobacter pylori bacteria. Coloured transmission electron micrograph (TEM) of a section of the surface of a human stomach infected with Helicobacter pylori bacteria (pink). Formerly known as Campylobacter pyloridis, these are spiral-shaped Gram-negative bacteria. Colonies of H. pylori are found in the mucus lining of the stomach. They cause gastritis, and are also the most common cause of stomach ulcers. H. pylori may also be a cause or co-factor for gastric cancer, as its presence increases the risk of developing stomach tumours. Magnification: x38,500 when printed 10 centimetres wide.