Helicobacter pylori bacteria. Computer artwork of Helicobacter pylori bacteria in a human stomach. Formerly known as Campylobacter pyloridis, these are spiral-shaped Gram-negative bacteria. The terminal flagella (hair-like structures) are used for locomotion. 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.