Heliobacter pylori, a tassled bacteria responsible for stomach ulcers, has been infecting humans for tens of thousands of years, and its genome has enabled German researchers to examine ancient patterns of human migrations.

Credit: Yutaka Tsutsumi/Wikimedia

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CANBERRA: The bacteria responsible for stomach ulcers infected modern humans before they left Africa 58,000 years ago, and could be used to trace ancient migration patterns.

More than half the world's population is infected with the bacteria Helicobacter pylori. The infection increases the likelihood of developing stomach cancer, as well causing painful stomach ulcers - a discovery that netted Australian scientists Barry Marshall and Robin Warren the Nobel Prize for medicine in 2005.

A new study, published today in the British journal Nature, settles a controversial debate surrounding the bacteria. Until today there was no way to tell whether infection of humans with H. pylori began only recently, or if it occurred prior to the 'Out of Africa' event, when modern humans left Africa and first colonised parts of Asia (see Out of Africa and into Russia, Cosmos Online).

"Our results establish that anatomically modern humans were already infected by H. pylori before their migrations from Africa and demonstrate that H. pylori has remained intimately associated with their human host population ever since," claimed the researchers, led by Mark Achtman of the Max Planck Institute for Infection Biology in Berlin, Germany.

Transmission of H. pylori from one person to another requires close contact. According to other medical studies, most people acquire the bacteria orally from their mother during infancy - with little transmission between unrelated individuals.

What this means, explained the researchers, is that families tend to carry the same strain of H. pylori. Passed on from generation to generation, eventually this leads to an entire local population carrying a distinct type of the bacteria.

In the study, the team collected 769 samples of H. pylori from 51 ethnic sources, encompassing Asia, Europe, Africa, the Middle East and South America. They sequenced seven sections of the DNA, totalling around 3,500 base pairs.

Multiple samples of H. pylori were taken from each location, so that the scientists could measure the genetic diversity. If a large number of unique sequences existed in one sample area, the local genetic diversity was considered high.

Genetic diversity within H. pylori was found to decrease with distance from east Africa. According to the team, this pattern in genetic diversity is also seen in the genomes of the world’s human population.

While genetic diversity decreases with isolation, genetic differences between populations increase with the distance between them. This has already been observed within human populations, but this study marked the first time it was seen within H. pylori.

"The key patterns in the distribution of H. pylori genetic diversity mirror those of its human host," the researchers explained.

They also discovered that the genetic makeup of H. pylori is actually more diverse than that of its human hosts. Because the patterns in each mirror the other, the origin of a specific human host can be determined by deciphering the genetic sequence of his or her H. pylori bacteria. Already, claimed the researchers, they can discern patterns that indicate prehistoric migrations.

Using a computer simulation based on the genetics and geographic locations of H. pylori, the researchers determined that east Africa was the likely source for the bacteria, that it migrated out of east Africa about 58,000 years ago (the same time as humans did), and that it spread from east Africa over the same time scale as the human population.

"We conclude that H. pylori accompanied anatomically modern humans during their migrations from Africa," the researchers reported in the paper. "H. pylori and human populations have been evolving intimately ever since."