According to a team of researchers, a bacterium that is found in almost half of all colon cancers is also found in their liver metastases. The link between the aggressiveness of the tumor and the presence of the bacteria in the tumor is proven.
A bacteria called Fusobacterium nucleatum is likely to play a role in causing or stimulating the growth of colon cancer because a new study, published in the journal Science, shows that an antibiotic directed against this microbe slows the growth of these same cancer cells in mice.
And this is not an isolated case of association between cancer and bacteria: another bacteria has also been discovered in pancreatic cancers. In both types of cancer, most tumors harbor bacteria, but only a small proportion of the tumor cells are infected.
Some tumors harbor bacteria
The discovery of Fusobacteria in colon cancers, bacteria normally present in the mouth, dates back to 2011. Since then, teams around the world have reported having found them in colon cancers, but without giving an answer to the questions that this discovery raised.
The new publication specifically studied colon cancers metastasized to the liver. Liver metastases, observed up to 2 years after the initial colon cancer surgery, were removed surgically and analyzed for bacteriological analysis. Metastases, from colon cancers infected with Fusobacteria, were also infected even after spread to the liver. In contrast, colon tumors that did not have the bacteria originally did not have them after spreading to the liver.
Bacteria travel with the tumor
The researchers also looked for bacteria in cancers that first appeared in the liver, not the colon. They didn’t find any. “By far the most likely explanation is that cancer metastasizes to the liver and carries this microbiome with it,” said Dr. Meyerson, of the Dana-Farber Cancer Institute and author of this study. “The bacteria aren’t there by accident,” he told The New York Times.
Dr Meyerson and his colleagues also transplanted infected colon cancers into mice, and these cancers have also increased. The team did this over and over, transplanting the cancers across four generations of mice, and it worked every time. Fusobacteria have remained attached to cancers.
The tumor microbiome has an influence
The researchers also treated the mice with an antibiotic, metronidazole, which destroys Fusobacteria, and the tumors then grew much more slowly. To have a control population, the researchers treated a group of mice with erythromycin, a related antibiotic to which Fusobacteria are not sensitive, and tumor growth was not affected.
This does not necessarily mean that all patients suffering from colon cancer should be treated with metronidazole to destroy any fusobacteria, on the one hand, because only half of colon cancers contain fusobacteria, d ‘on the other hand, because metronidazole does not act only on the fusobacteria involved, but also on other bacteria which can play an important modulating role. In addition, patients should take the antibiotic indefinitely, because Fusobacteria are constantly being reintroduced from the mouth.
A role in immune modulation
Thus, the researchers suggest that instead of directly causing colon cancer, Fusobacteria could modify the immune response of patients, as well as their reaction to treatments that use the immune system to destroy cancers. Another possibility is that the bacteria would act more directly by secreting chemicals that stimulate the local growth of cancer cells.
Ultimately, the accumulated evidence shows that Fusobacterium nucleatum promotes colon cancer or contributes to its development. A new example of the importance of the interactions that exist between bacteria and the human organism, whether via the immune system or another way.
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