They've discovered that roughly half of all human beings are infected with H. pylori--but that rates of infection vary enormously from place to place. In countries and communities with impure water, poor sanitation and crowding, infection rates are high--and higher, too, are incidences of stomach cancer and ulcers.
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Microbe's DNA Varies Greatly
Scientists also have snaked tubes into stomachs all over the world to study H. pylori more closely--and, by doing so, have found great genetic variability in the microbe's DNA. Even individual stomachs can contain dozens of genetically distinct bacteria, as if H. pylori is constantly reshaping its genome.
But there are clear, more global patterns also. Strains sampled from Asian stomachs, European stomachs and African stomachs differ distinctly--and the differences persist in the stomachs of offspring, even when they're now living in other parts of the world.
"Give me a bacterium and I can tell you generally where a person's ancestors came from," says Dr. David Graham, chief of gastroenterology at the Veterans Affairs Medical Center in Houston, one of the scientists who has made a study of this florid variability.
These global genetic signatures offer much more than the chance to perform a neat party trick: by comparing strains, scientists hope to track where and when H. pylori first took up residence in humans, and how it traveled to stomachs across the globe.
Though the details of H. pylori infection are unclear, it is believed to occur via the mouth, possibly via water, during the early years of life. And it clearly involves close community contact: in a study recently reported at a conference at UCLA, Himalayan Buddhists and Muslims who have lived in the same area for several hundred years retained genetically distinguishable populations of the microbe. But where did H. pylori come from? Is it a new or ancient resident in the human body?
Some researchers argue that H. pylori got into human stomachs hundreds of thousands, even millions of years ago. Others believe that the bug took up residence far more recently--perhaps caught from close contact with animals after the advent of agriculture, says Douglas Berg, professor of microbiology and genetics at Washington University School of Medicine in St. Louis.
