Illustration of a gastric (stomach) ulcer with a close-up view of Helicobacter pylori bacteria. Gastric, or peptic, ulcers are erosions of the mucosal lining of the stomach. Helicobacter pylori is a curved, or spiral, Gram-negative bacilli with flagella at one end that make it highly motile. Helicobacter pylori has recently been linked with the formation of stomach ulcers through observation of the bacterium in biopsy specimens from patients with active chronic gastritis and peptic ulcers.