By Patricia Reaney LONDON, Feb 7 (Reuters) - Humans have had an intimate relationship with the bacteria that causes painful stomach ulcers for more than 60,000 years, scientists said on Wednesday. A team of international researchers who traced the origins of a tummy bug known as Helicobacter pylori, which is also linked to stomach cancer, found it migrated out of Africa along with modern humans in their digestive system. "Humans and Helicobacter have evolved, or co-evolved, in a very intimate way with strains of bacteria having infected specific human populations for a very long time," said Dr Francois Balloux, of the University of Cambridge, who worked on the study. "We are quite confident that the infection is very old, something that predates the exit out of Africa," he told Reuters. The scientists from Britain, Germany, the United States, France, South Africa and Sweden uncovered the link -- which could improve understanding of the disease-causing bacteria and its virulence -- by analysing DNA sequence patterns from both the bacteria and its hosts. The findings are reported online by the journal Nature. By using computer simulation they showed how the bacteria spread around the world. The genetic differences they found in the bacteria mirrored those that occurred in human populations as they left Africa. So the spread of humans and Helicobacter pylori paralleled each other. Helicobacter pylori lives in the stomach and more than half of the world's population is infected with it. Although most people do not develop any illness, some develop ulcers in the stomach or a part of the small intestine known as the duodenum. Antibiotics are the usual treatment. Balloux said the findings could reveal whether long co-evolution between a disease and the host leads to increased or decreased virulence. "The research not only shows the likelihood that for tens of thousands of years our ancestors have been suffering the effects of this bacteria but it also opens up new possibilities for understanding human migration," he added. (Editing by Steve Addison, Reuters Messaging: patricia.reaney.reuters.com@reuters.net))
