Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria (yellow) on the surface of the human stomach. Formerly known as Campylobacter pyloridis, these are Gram-negative spiral-shaped 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: x14,000 when printed 10 centimetres wide.