Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria (green) on a cell from the lining 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.