You’re sitting in the dentist’s chair, nodding along to the familiar flossing lecture you’ve been politely ignoring for most of your adult life. Fair enough. It’s hard to get excited about gum health. But it turns out your dentist may have been underselling the pitch.
A study published in January 2026 in Cell Communication and Signaling shows that a common gum disease bacterium can promote breast cancer growth and spread in mice, and the findings hint at a particularly troubling link for people carrying BRCA1 mutations (1). “Floss to help prevent cancer” probably wasn’t on your 2026 bingo card, yet here we are.
A Familiar Microbe in an Unfamiliar Place
Fusobacterium nucleatum (try saying that three times fast) is a bacterium you’ve almost certainly hosted, whether you knew it or not. This anaerobic microbe is a key player in the oral microbiome, where it helps build biofilms: those slimy microbial communities that coat our teeth and tongues every day. In moderation, F. nucleatum is just part of life in the mouth. But when oral health breaks down and periodontal disease moves in, F. nucleatum throws a party.
The problem is, it doesn’t always stay at the party. Scientists have known for some time that this bacterium can travel. It’s been linked to colorectal cancer, where it can colonize tumors by latching onto a sugar molecule called Gal-GalNAc that’s overexpressed on cancer cells (2, 3). It’s also been detected in head and neck cancers and even the placenta (4), which is quite the travel itinerary for a microbe that’s supposed to live in your mouth.
So when oncology researcher Dipali Sharma and her team at the Johns Hopkins Kimmel Cancer Center analyzed patient datasets and found F. nucleatum lurking in malignant breast tumors, the natural question was: what’s it doing there?
Spoiler: nothing helpful.
From Mouth to Mammary: What the Mouse Studies Showed
The team, led by author Sheetal Parida, designed a series of experiments to test whether F. nucleatum was an active troublemaker in breast tissue or just an innocent bystander that happened to be in the wrong neighborhood.
First, they introduced the bacterium into the mammary ducts of healthy mice. The result: the animals developed inflamed, metaplastic and hyperplastic lesions. That’s not cancer, but it’s the kind of cellular misbehavior that can set the stage for it. Think of it as the “pre-gaming” phase. These lesions came with a side of inflammation, DNA damage and increased cell proliferation (1).
Then, the researchers took mice that already had small mammary tumors and injected F. nucleatum into their bloodstreams, mimicking the way the bacterium would naturally hitch a ride from the mouth to distant sites. Over six weeks, the tumors in bacterium-exposed mice grew to roughly threefold the size of tumors in control animals. And in a finding that no one was hoping for: cancer had spread to the lungs of every single mouse that received F. nucleatum (1).
The Mechanism: DNA Damage and Sloppy Repairs
So how does a mouth microbe fuel tumor growth in breast tissue? The team traced the mechanism to DNA damage, specifically the kind that gets patched up badly.
Exposure to F. nucleatum triggered DNA double-strand breaks in cells. To fix this, the cells turned to nonhomologous end joining (NHEJ), which is essentially the duct-tape-and-hope-for-the-best approach to DNA repair. Unlike the more careful homologous recombination pathway, NHEJ glues broken DNA ends back together quickly but sometimes introduces mistakes in the process (1).
A few errors here and there might not matter much. But when they accumulate, they can push cells further down the road toward malignancy. And that problem gets worse when you factor in genetics.
A Troubling Connection to BRCA1
In laboratory tests on human breast cancer cells, the researchers found that F. nucleatum was particularly good at colonizing cells carrying a BRCA1 mutation, as if the bacterium had a VIP pass.
This is concerning because BRCA1 normally plays a starring role in homologous recombination repair, the cell’s more precise method for fixing double-strand DNA breaks. When BRCA1 is mutated, that careful repair system is compromised, and cells become more reliant on error-prone backups like NHEJ (1).
In other words: cells that are already working with a weakened repair toolkit may be especially vulnerable when F. nucleatum shows up and starts breaking things. Sharma described it as multiple risk factors converging, with the bacterium acting as an environmental factor that cooperates with an inherited genetic mutation (5).
That said, this particular connection has only been demonstrated in cell culture so far. Whether the same heightened vulnerability plays out in living organisms, and ultimately in human patients, is still an open question.
Pump the Brakes (Just a Little)
Before you cancel all your plans and schedule an emergency dental cleaning, it’s worth noting what this study doesn’t tell us. The research was conducted in mice, and the bacterium was introduced via injection rather than through the natural oral-to-bloodstream route. We don’t yet know whether F. nucleatum can independently cause breast cancer or whether it only accelerates disease in combination with other factors.
As Firoozeh Samim, an oral medicine specialist at McGill University who was not involved in the study, pointed out: cancer is multifactorial. The bacterium may be one piece of a much larger puzzle, alongside genetics, environment and overall health, that together tip the balance toward disease (5).
Still, this work builds meaningfully on an already compelling trail of evidence. A 2020 study in Nature Communications showed that F. nucleatum could colonize breast tumors in mice using its Fap2 protein and suppress anti-tumor immune responses (2). The Sharma team’s own 2021 work in Cancer Discovery demonstrated that the bacterium could activate Notch and β-catenin signaling pathways, both implicated in tumor progression (6). The new study layers in cancer initiation, DNA damage mechanisms and the BRCA1 connection, adding real depth to this growing body of work.
The Takeaway: Your Mouth and the Rest of You Are on the Same Team
Perhaps the most practically interesting angle here is what it could mean for cancer prevention. If oral bacteria can hop a ride through the bloodstream and influence tumor development in distant tissues, then maintaining good oral health isn’t just about keeping your smile camera-ready. It could be a meaningful piece of the cancer-prevention conversation.
Samim emphasized this point, noting that continued research could build the case for integrating oral health care into broader cancer prevention strategies (5). It’s a concept that sounds deceptively simple (brush, floss, see your dentist) but the biology behind it is anything but.
For now, the research is still in its early stages. Human studies will be essential to determine whether these mouse findings translate to the clinic. But the thread connecting your mouth to the rest of your body continues to get thicker with every new study.
Your dentist, meanwhile, feels very vindicated.
References
Parida, S., Nandi, D., Verma, D., Yi, M., Yende, A., Queen, J., Gabrielson, K. L., Sears, C. L., & Sharma, D. (2026). A pro-carcinogenic oral microbe internalized by breast cancer cells promotes mammary tumorigenesis. Cell communication and signaling : CCS, 10.1186/s12964-025-02635-9. Advance online publication. https://doi.org/10.1186/s12964-025-02635-9
Parhi, L., Alon-Maimon, T., Sol, A. et al. Breast cancer colonization by Fusobacterium nucleatum accelerates tumor growth and metastatic progression. Nat Commun 11, 3259 (2020). https://doi.org/10.1038/s41467-020-16967-2
Little, A., Tangney, M., Tunney, M. M., & Buckley, N. E. (2023). Fusobacterium nucleatum: a novel immune modulator in breast cancer?. Expert reviews in molecular medicine, 25, e15. https://doi.org/10.1017/erm.2023.9
Parhi, L., Abed, J., Shhadeh, A., Alon-Maimon, T., Udi, S., Ben-Arye, S. L., Tam, J., Parnas, O., Padler-Karavani, V., Goldman-Wohl, D., Yagel, S., Mandelboim, O., & Bachrach, G. (2022). Placental colonization by Fusobacterium nucleatum is mediated by binding of the Fap2 lectin to placentally displayed Gal-GalNAc. Cell reports, 38(12), 110537. https://doi.org/10.1016/j.celrep.2022.110537
Rosen, M. Gum disease bacteria can promote cancer growth in mice. Science News. February 4, 2026. https://www.sciencenews.org/article/gum-disease-bacteria-breast-cancer
Parida, S., Wu, S., Siddharth, S., Wang, G., Muniraj, N., Nagalingam, A., Hum, C., Mistriotis, P., Hao, H., Talbot, C. C., Jr, Konstantopoulos, K., Gabrielson, K. L., Sears, C. L., & Sharma, D. (2021). A Procarcinogenic Colon Microbe Promotes Breast Tumorigenesis and Metastatic Progression and Concomitantly Activates Notch and β-Catenin Axes. Cancer discovery, 11(5), 1138–1157. https://doi.org/10.1158/2159-8290.CD-20-0537
Sara Millevolte
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