Australians Barry Marshall and J. Robin Warren have won the 2005 Nobel Prize in physiology or medicine. The researchers discovered the bacterium Helicobacter pylori, and explored its role in gastritis and peptic ulcers.

The prize for medicine is selected in Stockholm, Sweden by the Karolinska Institute. It is the first Nobel to be awarded this year, and will be followed by physics, chemistry, literature, peace and economics.

Two Canadians were believed to be finalists for the prestigious award. Ernest McCulloch and James Till recently won the Albert Lasker Foundation award for basic research for their pioneering work with stem cells.

The process for selecting winners is extremely secretive -- nominations are kept sealed for 50 years -- though often winners of the Lasker prize go on to receive the Nobel.

Helicobacter pylori is a bacterium that appears in the stomach in about 50 per cent of all humans. It is generally thought to be acquired in childhood. Only a minority of infected individuals go on to develop stomach disease.

Until 1982, stress and lifestyle were considered the major causes of peptic ulcer disease. Even though stomach ulcers could be healed by inhibiting gastric acid production, patients frequently relapsed.

Marshall and Warren discovered that the small curved bacteria colonized the lower part of the stomach in almost all patients studied with gastric inflammation, duodenal ulcer or gastric ulcer. They then further showed that patients could only be properly cured of gastric ulcers when H. pylori was eradicated from the stomach.

It is now firmly established that Helicobacter pylori causes more than 90 per cent of duodenal ulcers and up to 80 per cent of gastric ulcers.

Many diseases in humans such as Crohn's disease, ulcerative colitis, rheumatoid arthritis and atherosclerosis are due to chronic inflammation. The discovery that one of the most common diseases of mankind, peptic ulcer disease, has a microbial cause, has stimulated the search for microbes as possible causes of other chronic inflammatory conditions.

The Nobel prize will be awarded Dec. 10 at a ceremony in Stockholm.