Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of H. pylori bacteria (green) on human gastric epithelial cells (orange). Formerly known as Campylobacter pyloridis, this is a Gram-negative spiral-shaped bacterium. It is motile, possessing extensions, called flagella, that are used for propulsion. H. pylori is found in the mucus lining of the stomach. It causes gastritis, and is also the most common cause of stomach ulcers. It may also be a cause or co-factor for gastric cancer as its presence increases the risk of developing stomach tumours.