Helicobacter pylori bacterium. Coloured transmission electron micrograph (TEM) of an Helicobacter pylori bacterium. Formerly known as Campylobacter pyloridis, this is a Gram-negative spiral-shaped bacterium. It is motile, possessing extensions, called flagella, that are used to propel the cell. H. pylori is found in the mucus lining of the stomach. It cause gastritis, and is 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.