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Certain bacteria in the mouth may cause colorectal cancer, a new study suggests.

Colorectal cancer is the second leading cause of cancer-related death in the United States, and recent studies have shown that gut microbes known as fusobacteria, which are found in the mouth, are also prevalent in tissues from colorectal cancer patients. However, scientists were unsure whether these microbes directly contribute to the formation of tumors.

Now, two studies published in the journal Cell Host & Microbe reveal that fusobacteria can stimulate bad immune responses and trigger cancer growth genes to generate colorectal tumors. Researchers said the latest findings could lead to more effective strategies for early diagnosis, prevention and treatment of colorectal cancer.

"Fusobacteria may provide not only a new way to group or describe colon cancers but also, more importantly, a new perspective on how to target pathways to halt tumor growth and spread," senior study author Wendy Garrett, from the Harvard School of Public Health and the Dana-Farber Cancer Institute, explained in a news release.

Researchers in one study found that fusobacteria are abundant in human adenomas, which are benign tumors that can become malignant over time.  After experimenting on mice, researchers found that fusobacteria accelerated the formation of tumors by attracting immune cells called myeloid cells, which invade tumors and trigger inflammatory responses that can cause cancer.

Another study revealed that fusobacteria rely on a molecule found on the surface of these bacterial cells to attach to and invade human colorectal cancer cells. These molecules, called Fusobacterium adhesin A (FadA), then trigger cancer growth genes and stimulate inflammatory responses in these cells to promote tumor formation. Researchers in the second study also found that FadA levels were significantly higher in tissues from patients with adenomas and colorectal cancer compared with healthy individuals.

"We showed that FadA is a marker that can be used for the early diagnosis of colorectal cancer and identified potential therapeutic targets to treat or prevent this common and debilitating disease," Yiping Han of Case Western Reserve University School of Dental Medicine said in a news release.