A bacterium best known for causing sore, bleeding gums may also settle in the breast, where it appears to damage DNA and accelerate tumor growth.
That is the finding of a Johns Hopkins Kimmel Cancer Center study published January 15, 2026 in Cell Communication and Signaling. The team tracked Fusobacterium nucleatum as it moved from the mouth through the bloodstream into breast tissue.
In animal models and human cancer cells, exposure to the microbe triggered inflammation, DNA damage, and abnormal cell growth. It also activated an error-prone DNA repair pathway called nonhomologous end joining and raised levels of a protein tied to more aggressive tumor behavior, greater invasion, and resistance to chemotherapy.
The effect was significantly stronger in cells carrying a BRCA1 mutation. These cells expressed higher levels of a surface sugar called Gal-GalNAc, which acts like a docking site for the bacterium.
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As a result, they absorbed more Fusobacterium nucleatum and retained it across multiple cell generations, intensifying the tumor-promoting effects.
"The key takeaway is that this oral microbe can reside in breast tissue and that there is a connection between this pathogen and breast cancer," said Dipali Sharma, professor of oncology at Johns Hopkins.
What the study actually shows
The findings are preclinical. They come from mouse models and cultured human breast cancer cells, not from patients, so the mechanism has been mapped in the laboratory but a causal link in humans has not yet been established.
The research team also said more studies are needed before oral health can be considered a modifiable risk factor in breast cancer prevention or management.
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Still, the mechanism is striking. When the bacterium was introduced into breast ducts, it produced early tissue changes described as metaplastic and hyperplastic lesions, noncancerous shifts in which cells multiply too much or transform into a different cell type.
In parallel experiments, the microbe accelerated tumor growth and increased spread to the lungs.
"Nothing happens in isolation. The results suggest that multiple risk factors come together with F. nucleatum acting as an environmental factor that may cooperate with inherited BRCA1 mutations to promote breast cancer and tumor aggressiveness," Sharma said.
Why gum care matters anyway
Whatever the outcome of the breast cancer question, gum disease is worth taking seriously on its own. Poor gum health has already been linked to other systemic conditions, including cardiovascular disease.
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The NIH's National Institute of Dental and Craniofacial Research recommends brushing twice a day with fluoride toothpaste, cleaning between teeth daily with floss or another interdental device, and keeping up routine dental checkups and professional cleanings.
Tobacco use, the institute notes, is the single most significant risk factor for gum disease.
Anyone with persistent bleeding or receding gums should raise it with a dentist rather than self-diagnose, and that is especially worth doing for people with a BRCA1 mutation or a strong family history of breast cancer.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.