Helicobacter pylori bacteria and yeast in the stomach. Animated coloured scanning electron micrograph (SEM). H. pylori is a Gram-negative spiral-shaped bacterium (green) found in the mucous lining of the stomach (blue) often in conjunction with other microbes including yeast (red). 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.