Helicobacter pylori
Helicobacter pylori is a gram-negative bacterium that lives inside the mucus lining of the stomach. It belongs among the proteobacteria. Many gastric ulcers are caused by H. pylori infection, although most humans are infected and not all infections lead to symptoms. Two strains of H. pylori have been sequenced.
Information on ulcers and H. pylori at the Centers for Disease Control and Prevention.Links