Spiral organisms: what are they? A microbiologic introduction to Helicobacter pylori.

The stomach provides a hostile environment to most microorganisms owing to the antibacterial effect of gastric acidity. However, over the period of mammalian evolution a highly specialized group of bacteria has evolved to inhabit the gastric mucosa. The common features of these bacteria that allow survival in the stomach and colonization of their natural ecologic niche, mucus, are a specialized form of motility, microaerophilism, and very high activity of the enzyme urease. Other important adaptations have been the acquisition of sophisticated mechanisms to evade host defences, and in certain cases tissue damage may result. The most important of these usually spiral-shaped bacteria is Helicobacter pylori. It is proposed that this bacterium is the human gastric spirillum that in most persons lives in harmony with its natural host, resulting in asymptomatic infection. However, if as yet undefined conditions are present, serious disease may result-that is, peptic ulceration. A consequence of long-term infection may be atrophic gastritis, which sets the scene for gastric carcinoma. The more the microbiology of the stomach is studied, the more species of Helicobacter are discovered, such as H. mustelae in the ferret, H. felis in cats and dogs, and 'Gastrospirillum hominis' in primates, cats, dogs, and pigs. These other helicobacters provide us with a wide spectrum of animal models of gastroduodenal disease. To understand mechanisms of pathogenesis, it is suggested that systematic study of all members of the genus will identify important colonizing factors and provide a realistic perspective on the putative virulence factors that have been proposed for H. pylori to date.