Nearly two thirds of multiple sclerosis patients carrying high levels of a common oral bacterium have trouble walking. Compare that to fewer than one in five MS patients with lower levels of the same microbe, and the tongue starts to look less like neutral territory and more like a window into disease severity.
The bacterium is Fusobacterium nucleatum, best known for its role in gum disease. But researchers at Hiroshima University found something unexpected when they swabbed the tongues of 56 MS patients: the amount of F. nucleatum in someone’s mouth corresponded closely with their score on standard disability measures. More of the bacterium meant worse outcomes, even after accounting for how long someone had been sick or how many relapses they’d experienced.
Multiple sclerosis is an autoimmune disease where the body attacks myelin, the protective coating around nerve fibers. That damage accumulates over time, impairing movement and cognition. What triggers it remains murky, though genetics, smoking, and certain viral infections all seem to play a role.
Why this bacterium, why MS
The study, published in Scientific Reports, included 98 patients with various inflammatory brain diseases. The researchers used quantitative PCR to measure four different periodontal bacteria, but only F. nucleatum showed a link to disability, and only in MS patients.
People with neuromyelitis optica spectrum disorder, another autoimmune condition that damages the central nervous system, showed no such pattern. That specificity matters. It suggests F. nucleatum isn’t just tagging along with inflammation generally but may be interacting with MS in a particular way.
“Because the oral cavity is a major source of chronic inflammation, clarifying its relationship with multiple sclerosis severity is important,” Masahiro Nakamori, Associate Professor at Hiroshima University Hospital, explains.
The mechanism isn’t proven, but chronic oral inflammation could be sending persistent signals to an already hyperactive immune system. F. nucleatum is sometimes called a bridge bacterium because it physically connects different bacterial communities in dental biofilms. In practice, that means it may help other pathogens colonize and thrive, creating a more inflammatory environment overall.
When the researchers looked at patients with high levels of F. nucleatum alongside other periodontal pathogens, the association with severe disability got stronger. The odds were roughly tenfold higher compared to patients with lower bacterial loads.
Oral health as a lever
This was a small, cross-sectional study. It can’t prove that oral bacteria cause MS disability, only that the two correlate. But frankly, correlation at this level is worth investigating further, especially when the factor in question is something patients and doctors can actually address.
The team is planning larger, multi-center trials to confirm the findings and explore whether treating gum disease or improving oral hygiene could slow disability progression. If it does, neurologists may start asking about dental visits as routinely as they ask about relapses.
For now, the work leaves a provocative question: could something as mundane as brushing your teeth influence the course of a neurological disease? The study doesn’t answer that. But it suggests the mouth deserves more attention than it’s gotten.
Scientific Reports: 10.1038/s41598-025-22266-x
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