The bacterium that causes stomach ulcers might not have been with humans forever, a new study suggests, contradicting a long-held assumption. Comparing pieces of DNA from Helicobacter pylori, scientists discovered that strains from Peru resemble those from Spain and not those from eastern Asia.
"My favorite interpretation of this finding is that the Spanish brought H. pylori to Peru when they conquered the Incan empire nearly 500 years ago and that the bacterium was not present in the ancestors who crossed the Bering Strait from Asia more than 10,000 years ago," says Douglas E. Berg, Ph.D., the Alumni Professor of Molecular Microbiology and professor of genetics at Washington University School of Medicine in St. Louis.
Berg and collaborators in Britain, China, Guatemala, India, Japan, Peru, South Africa, Spain, Sweden and the United States report their findings in the June issue of the Journal of Bacteriology.
H. pylori is carried by more than half the world's population, and it can thrive in the stomach for years. Whereas some people suffer no apparent consequences, others develop peptic ulcer disease. Gastric cancer, the leading cause of cancer deaths in some developing countries, also has been associated with H. pylori.
Analyzing DNA from more than 500 strains from five continents, Berg's group focused mainly on a region called the cag pathogenicity island. One part of this region contains apparently vestigial genes, and it varies in size because some strains have lost pieces of DNA whereas others have inserts. Also, some base pairs, the building blocks of DNA, have been substituted for others.
As well as measuring the size of the vestigial segment, the researchers examined the DNA sequences of cagA, a gene that lies just next to it, and vacA, which lies elsewhere in the chromosome. The cagA gene codes for a protein that, when phosphorylated, alters the internal communication system of human cells, whereas vacA cod
Contact: Linda Sage
Washington University School of Medicine