Gastric bacteria. Coloured scanning electron micrograph (SEM) of Helicobacter pylori (green) and Lactobacillus fermentum (blue) bacteria. H. pylori is found in the mucus 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. L. fermentum has been shown to form clumps with H, pylori, disabling the pathogenic bacteria. Probiotic L. fermentum could be used to treat H. pylori infections. Magnification: x5000 when printed at 10 centimetres wide.