Helicobacter pylori is a bacterium that infects the mucus lining of the human stomach. It belongs among the proteobacteria. Many peptic ulcers and some types of gastritis are caused by H. pylori infection, although most humans are infected and many infections do not lead to any symptoms. Treatment of these diseases now often includes antibiotics. Long term infection with the bacterium may also be associated with gastric cancer.

About 2/3 of the world population are infected by the bacterium. Under poor sanitory conditions, one commonly finds infected children; in the U.S, older people (about 50% of those over the age of 60, 20% of those under the age of 40) and poor people are more likely to be infected. The infection apparently persists for life; the immune system cannot fend of the organism. The bacteria have been isolated from feces, saliva and dental plaque of infected patients, which suggests possible transmission routes. One can test for H. pylori infection with blood antibody tests, breath tests (where the patient drinks 14C or 13C labeled urea, which the bacterium metabolizes to carbon dioxide that can be detected in the breath), or endoscopy.

Helicobacter pylori
H. pylori is a spiral-shaped gram-negative bacteria, about 3 micrometers long with a diameter of about 0.5 micrometers. It has 4-6 flagella. It is microaerophilic, i.e. it requires oxygen but at lower levels than those contained in the atmosphere. Several strains are known, and the genomes of two have been completely sequenced.

With its flagella and its spiral shape, it drills through the mucus layer of the stomach and attaches to epithelial cells. It contains the enzyme urease which converts urea into ammonia and bicarbonate. The ammonia is useful to the bacterium since it partially neutralizes the very acidic environment of the stomach (whose very purpose is to kill bacteria). Ammonia is however toxic to the epithelial cells.

The bacterium and its link to peptic ulcers was described first in 1982 by two Australian researchers: Robin Warren and Barry Marshall. Before the discovery, stress or diet were considered the primary causes of these ulcers. They were treated with drugs that neutralize stomach acid or decrease its production. While this worked well, the ulcers very often reappeared. Acceptance in the medical community of the role of H. pylori was slow. In 1994, the NIH conducted a conference which ended with a consensus statement affirming the causative role of H. pylori in these diseases.

The bacterium was initially called Campylobacter pyloridis, then C. pylori and finally placed in its own genus Helicobacter. Some other species of that genus have now been identified in the stomachs of other mammals and some birds.