“There are more and more papers showing how they can be part of carcinogenesis,” says Straussman. Bacteria can also be alter the ability of the immune system to attack and kill cancer cells, he adds. “But we’re really scratching the surface here.” Much more needs to be done, he says, to study the effects that bacteria inside tumors have on the course of cancers.
There are already some clues. For example, a 2022 study by scientists in China suggests that some bacteria in breast tumors could make it easier for cancer cells to spread to other parts of the body. The researchers found bacteria living inside the breast tumor cells circulating in the blood of the mice. These circulating cancer cells they break away from the primary tumor and can travel to other parts of the body, where they can metastasize and grow. However, as tumor cells rattle around in the bloodstream, they are exposed to stress that causes some of them to rupture.
The Chinese researchers found that the microbes inside these mobile tumor cells appear to protect them from some of the stress they experience. They seem to do this by helping to rearrange internal cellular support structures known as the cytoskeleton so the cells are more robust. When the scientists removed these bacteria from the mouse tumors, the tumors appeared to lose their ability to metastasize, even though the primary breast cancer continued to grow.
“There is increasing evidence that specific microbes in the gut, skin and other mucosal organs, as well as in tumors, can either promote tumor growth and progression or alternatively antagonize it,” says Douglas Hanahan, M.D. from the Swiss Institute. for Experimental Cancer Research in Lausanne, Switzerland, and the author of Hallmarks of Cancer: New Dimensions. However, the outlook remains cloudy. “The picture is very complicated, and although there are clues, there is no definitive clarity on who does what.”
Other studies analyzed Fusobacterium nucleatum, an oral bacterium associated with gum diseasebut it could also be associated with various types of cancer. Seems these bacteria can migrate from the mouth to the colorectal cancer cell through the bloodstream. Each bacterium carries specific particles on its surface that bind to the surface of cancer cells, allowing it to colonize them.
Once established, the bacteria can speed up the growth and spread of tumors by hampering the immune system’s ability to kill cancer cells. A protein produced by Fusobacterium nucleatum it binds to a molecular checkpoint mechanism on the surface of human natural killer cells and T cells, which are key parts of the immune system’s defenses against tumors. this union inhibits the ability of cells to kill cancer cells. Bacteria also deploy a molecular arsenal that makes cancer cells more resistant to chemotherapy.
Besides, Fusobacterium nucleatumDNA found in human breast cancer samples. This suggests that it also affects tumors in other parts of the body. In one study, when the bacteria were introduced into mice with breast cancer, It accelerated the progression and spread of the disease.. Giving the mice antibiotics prevented this.
It may seem tempting to include antibiotics in cancer therapies, but it’s not as simple as that. Many of the microbes in our bodies are benign or even beneficial, so a brute-force antibiotic treatment could do more harm than good, Hanahan says.
Instead, researchers must try to unravel the full complexity of the tumor-associated microbiome. Entire communities of microbes can be found inside tumors, and they support each other in unexpected ways.
