Fusobacterium nucleatum

Fusobacterium nucleatum is a bacterium that normally lives in the mouth. It isn't alone there: it shares that space with lots of other microorganisms, living things too small to see with the naked eye. And among them it plays a special role. It works as a "bridge," because it helps other microorganisms stick together.

Think of the mortar between the bricks in a wall, which helps hold them to one another. This bacterium does something similar, and what comes out of all that sticking together is dental plaque, the sticky substance that causes cavities, the holes that form in teeth. That plaque can also contribute to gum disease.

The rest of the story happens far from the mouth, in the intestines. In people with colorectal tumors, meaning tumors of the colon and rectum, the last stretch of the intestines, this bacterium has been found in greater numbers than in people without cancer. Researchers have flagged it as a possible contributor to the growth of colorectal cancer, in other words, as something that might help it progress. What still isn't clear is its role at the very start, when a tumor is just beginning to form.