The microbiology and epidemiology of Helicobacter pylori infection.

Helicobacter pylori is part of a genus of specialized bacteria that have adapted to the ecological niche provided by gastric mucus. H. pylori has exploited the human niche, while further species of Helicobacter have inhabited the gastric mucosa of other animals. The preferred habitat of H. pylori is the gastric antrum. In humans with normal gastric function, the organism is mainly restricted to the antral surface, where a number of specialized traits allow it to flourish, while causing minimal harm to its host. These include a characteristic motility that allows it to swim rapidly through viscous mucus, and the ability to manufacture large amounts of the enzyme urease. This enzyme breaks down endogenous urea to form ammonia, which protects the bacterium from gastric acidity. Specific adhesions bind a number of the bacteria to the gastric surface, some swim freely in the mucus, and others possibly endocytose into the epithelial cells. It is probably these inaccessible colonization sites that make the organism so difficult to eradicate. In some patients, the normally harmless balance between host and bacterium is disturbed, resulting in peptic ulceration. Modifications to the mucus or epithelial surface in the proximal duodenum, towards the gastric phenotype, make the tissue more susceptible to H. pylori infection of the duodenum by spread of organisms from the antrum. Gastric acid output becomes further increased and the duodenal mucosa is rendered more susceptible to acid attack, leading to peptic ulceration. In other situations, the level of inflammation is enhanced and immunopathology results, followed in the longer term in some cases by atrophy and gastric cancer.(ABSTRACT TRUNCATED AT 250 WORDS)