Helicobacter pylori bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori bacteria, a common cause of gastritis (stomach lining inflammation). They are Gram-negative and rod-shaped, and move with a helical motion. They are found in the mucous lining of the stomach, commonly in middle-aged people. Their presence is also associated with the formation of stomach and duodenal ulcers. Magnification unknown.