Scientific American has new research that finds a connection between one subspecies of a bacterium commonly found in our mouths, Fusobacterium nucleatum, and the growth colon cancer, an ailment that’s currently on the rise:
F. nucleatum, associated with dental plaque and gingivitis, occurs naturally in the mouth microbiome. A decade ago scientists discovered that the bacterium was also found in colon cancer more often than in normal colon tissue. “This was particularly interesting because this microbe in noncancerous individuals is usually not present below the [mouth],” says the new study’s co-senior author Susan Bullman, a biologist at the Fred Hutchinson Cancer Center.
To further explore the microbe’s relationship to colon cancer, Bullman and her colleagues conducted extensive sequencing on F. nucleatum within colon cancer tumors and looked at how the microbe influenced the intestinal environment. The team first analyzed the genomes of F. nucleatum found in colon tumors in order to compare them with those found in the mouth. It collected colon tumors from approximately 100 people and then broke up the tumors and placed them on agar plates to allow the microbes present to grow.
After isolating the F. nucleatum from these cultures, the scientists performed a process called long-read sequencing to get a comprehensive look at the bacterium’s genome.
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This revealed two main clades within a subspecies (called F. nucleatum animalis) that were distinguished by differences in DNA bases and patterns of encoded proteins. Bacteria in the two clades also had distinct appearances under the microscope: specimens in the second clade were longer and thinner than those from the first.
F. nucleatum animalis from the colon tumors fell overwhelmingly into the second clade. This clade’s genomes seemed to code for characteristics that would help the bacteria survive the perilous journey from the mouth to the intestine—such as the ability to gain nutrients in hostile environments (such as an inflamed gastrointestinal tract) or to better invade cells.
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They gave one group of mice a single oral dose of F. nucleatum animalis from clade 1 and another a dose of clade 2 and then counted the number of tumors that formed. Mice in the clade 2 group developed a significantly higher number of large intestinal tumors in comparison with those given clade 1 bacteria or a nonbacterial control.
When the scientists measured metabolic molecules inside tumors from the mice with clade 2 bacteria, they found more molecules associated with cellular damage from oxidative stress, cancer cell division and inflammation than mice in the control and clade 1 bacteria groups. “This supports the idea that clade 2 bacteria are contributing to this proinflammatory, pro-oncogenic environment,” [study lead auther Martha] Zepeda-Rivera says.
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You can read more of the bacterial cancer research here, in Nature.