“The bacteria that live on your teeth are pretty much entirely different from the bacteria that live on your tongue,” says Jessica Mark Welch, a biologist at the ADA Forsyth Institute who specializes in the oral microbiome. “And there’s a different group of bacteria on the roof of your mouth and on your gums.”
In people with good oral health, the microbiome has a diverse community of beneficial microbes, including Streptococcus and Rothia, which help control inflammation and fend off pathogens. But that balance is sensitive. Changes in diet, oral hygiene, or overall health can quickly shift the microbial mix.
“Bacteria always respond to their environment,” Mark Welch explains. “If the mouth is exposed to a lot of sugar, then bacteria that thrive on simple sugars become more abundant.”
One of the best-known culprits is Porphyromonas gingivalis, a bacterium closely linked to periodontal, or gum, disease. While most bacteria prefer carbohydrates, P. gingivalis thrives on protein—quite literally feeding on gum tissue.
The recent JAMA Oncology study found that P. gingivalis and two other periodontal pathogens, E. nodatum and P. micra, as well as the common oral fungus Candida, were associated with a heightened risk of pancreatic cancer.
In this colored scanning electron micrograph (SEM), mixed oral bacteria adhere to buccal cells from the cheek. Researchers are increasingly exploring how these microbes influence health beyond the mouth. Steve Gschmeissner, Science Photo Library
The mouth–body connection
Chronic inflammation is a well-established risk factor for cardiovascular disease, type 2 diabetes, several cancers, and Alzheimer’s disease.
People with periodontal disease are three to five times more likely to develop cancers linked to the oral microbiome. “The hypothesis is that they develop a pathogenic type of inflammation rather than a healthy one,” Freire says.
Blood vessels under the tongue, cheeks, and other oral sites allow bacteria to migrate through the body, Freire adds. The immune system typically intercepts these bacteria, but some slip through, potentially leading to disease.
Fusobacterium nucleatum, which is typically found in the mouth, is an example of a pathogen that can promote tumor growth through migration. Patients with colorectal cancer were five times more likely to carry F. nucleatum in their stool than healthy counterparts, according to research from the National Cancer Institute.
Saliva offers another pathway for bacteria to travel through the digestive tract. In animal studies, P. gingivalis that migrated from the mouth has been shown to cause pancreatic cancer, according to a 2024 study.