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Gut Bacteria "Mismatch" Could Increase Gastric Cancer Risk

APR 11, 2017 | CAROLE ELLIS
If your gut bacteria originally immigrated to the “New World” with Christopher Columbus, but you are of Native American descent (either North, South, or Central America), then you could have a higher risk of developing stomach cancer and you might already have ulcers thanks to your gastrointestinal ethnic “mismatch.”
 
According to research published in February in PLOS Genetics, not only does the bacterium Heliobacter pylori evolve rapidly and somewhat problematically within the gut during what the researchers describe as “times of demographic flux,” but it can also spread to different ethnicities and form subpopulations that are distinct from all previous generations of the bacterium. Although humans and H. pylori have lived together, literally, for hundreds of years in relative harmony despite the bacterium’s tendency to cause stomach ulcers in a certain subset of the population, when the H. pylori in your stomach and your ethnic heritage do not coordinate, it can make for gastrointestinal trouble.
 
The researchers, led by Kaisa Thorell, MSc, PhD, an expert in gastroenterology, infectious diseases, and bioinformatics at the Karolinska Institutet in Solna, Sweden, and Koji Yahara, PhD, a senior research fellow in the Department of Bacteriology at the National Institute of Infectious Diseases in Japan, are involved in the analysis of more than 400 H. pylori genome sequences collected from North America, Central America, and South America. “In 1492, Christopher Columbus initiated a rapid colonization of the New World, principally by European migrants and Africans brought as slaves, that had catastrophic consequences for the indigenous population,” the team explained. They went on to note that in addition to diseases like smallpox, which get many historical mentions for the decimation of indigenous tribes in the Americas, “new populations of the stomach-colonizing bacterium H. pylori” colonized the New World as well.
 
The researchers classified bacterial populations of H. pylori that were likely present in the Americas before 1492 as hspAmerind, and noted that these were likely subpopulations of hspEAsia, which are found in China and Japan. The hpEurope bacteria were defined as a population of H. pylori from Europe that are likely “ancient hybrids” of the bacterium from Northeast Africa and Central Asia. In many cases, the team observed that the “immigrating” strains of the bacteria actually drove local strains close to extinction. “Bacteria can spread if they transmit particularly well or if they outcompete other strains in the same stomach,” Daniel Falush, PhD, senior author on the study and a professor at the University of Bath in the United Kingdom, told Contagion®. He added, “There are a number of reasons why Native American bacteria may be particularly bad [at this]…that might mean that they lack variation and are less able to compete.”
 
Dr. Falush noted that the group is presently looking at variations within multiple bacterial populations in order to confirm this. All this bacterial genealogy led the team to the present, wherein Latin America has some of the highest mortality rates linked to gastric cancer, the world over. “The mortality rates vary in different regions,” noted the researchers, speculating that “discordant origin of bacteria and host” could provide the link to high rates of cancer in some of these areas. H. pylori is already associated with increased risks of some types of gastric cancer because of its tendency in some hosts to cause ongoing irritation of the stomach lining, where the bacterium tends to “burrow” when the conditions of the stomach become too acidic.
 
The discordancy between bacterium and host was likely initiated on a large scale when European settlers and their African slaves arrived on the North and South American continents. “We found [that] bacteria of African origin may have been particularly effective in colonizing the new continent,” the team wrote, noting that H. pylori “can undergo high levels of recombination during mixed infection,” which could lead to an ancestry profile that is somewhat misleading because it may end up reflecting a local gene pool rather than the continent of origin. However, “recombination has not proceeded this far anywhere in the Americas and multiple populations with distinct ancestry profiles are found in most locations.” That being said, with some estimates indicating that as much as 84.7% of the adult population in Latin American countries has an H. pylori infection, the odds of some significant portion of that infected population having the genetic “mismatch” between human and bacterium is probable.
 
In future research, the group hopes to evaluate levels of virulence in light of bacterial and host genomics, said Dr. Falush. “One direction we have not covered is how bacterial genotype within admixed populations relates to human genotype,” he said. “For example, we know that there is a successful population of African origin within the US, but we do not know how common it is within white or black Americans. We did not have data on human ethnicity within the U.S. in this study, but expect to get more data like that.”
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