Helicobacter pylori is a micro-aerophilic, Gram-negative, slow-growing, spiral-shaped and flagellated organism. Its most characteristic enzyme is a potent multisubunit urease that is crucial for its survival at acidic pH and for its successful colonization of the gastric environment, a site that few other microbes can colonize. H. pylori is probably the most common chronic bacterial infection of humans, present in almost half of the world population. The presence of the bacterium in the gastric mucosa is associated with chronic active gastritis and is implicated in more severe gastric diseases, including chronic atrophic gastritis (a precursor of gastric carcinomas), peptic ulceration and mucosa-associated lymphoid tissue lymphomas. Disease outcome depends on many factors, including bacterial genotype, and host physiology, genotype and dietary habits.

Because of its importance as a human pathogen, our interest in its biology and evolution, and the value of complete genome sequence information in drug discovery and vaccine development, we have sequenced the genome of a representative H. pylori strain by the whole-genome random sequencing method. The genome consists of a circular chromosome with a size of 1,667,867 base pairs, which contains 1590 predicted coding regions.