Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria (green) in the stomach. H. pylori is a Gram-negative spiral-shaped bacterium found in the mucous 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. Magnification: x5000, when printed 10cm wide.