A recent study suggests a possible link between the presence of Fusobacterium nucleatum, a bacterium found in the mouth, and increased disease severity in patients with multiple sclerosis (MS). The research was published in Scientific Reports on November 3, 2025.
Multiple sclerosis is a chronic autoimmune disorder that affects the central nervous system by damaging the myelin sheath, which protects nerve cells. While its exact cause remains unknown, several factors such as viral infections, smoking, vitamin deficiencies, and genetics are believed to play a role. In Japan, the prevalence of MS has been rising since the 1980s, potentially due to environmental changes. Research has traditionally focused on gut microbiome alterations but is now also considering oral microbiota as influential in central nervous system diseases.
Periodontal disease is a widespread chronic bacterial infection that leads to persistent inflammation and can result in tooth loss. It affects an estimated 40 to 60 percent of people globally and has been linked to other health conditions like atherosclerosis, diabetes, and rheumatoid arthritis.
In this study, researchers collected tongue coating samples from patients with MS and related conditions—neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD). They analyzed whether higher levels of specific bacteria correlated with greater disability among these patients.
The results showed that MS patients with high levels of Fusobacterium nucleatum had significantly greater disability according to the Expanded Disability Status Scale (EDSS), compared to those with lower levels. This association was not seen in patients with NMOSD or MOGAD.
"This association was not observed in neuromyelitis optica spectrum disorder or myelin oligodendrocyte glycoprotein antibody-associated disease, suggesting a potentially multiple sclerosis-specific 'oral–brain axis' through which oral inflammation may influence neuroinflammatory disease severity," said Hiroyuki Naito, an assistant professor at Hiroshima University Hospital.
Researchers adjusted for various clinical factors such as age, duration of illness, number of attacks, and MS subtype. Even after these adjustments, they found that high levels of Fusobacterium nucleatum were linked to about ten times higher odds of severe disability among MS patients.
They reported that nearly two-thirds (61.5%) of MS patients with high levels of this bacterium had moderate-to-severe disability (EDSS score 4 or above), while only about one-fifth (18.6%) with milder disease showed similar bacterial abundance. No comparable pattern was seen among NMOSD or MOGAD patients. Additionally, MS patients who had both Fusobacterium nucleatum and another periodontal pathogen experienced even more severe disability.
"Fusobacterium nucleatum may act as a hidden 'bridge bacterium'-not only bridging bacterial communities in dental biofilms, but also potentially linking oral inflammation to neurological disability," said Nakamori.
The research team plans further studies involving larger groups across multiple centers to confirm these findings. They also intend to explore how oral pathogens might affect immune responses in MS through mechanistic analyses such as cytokine profiling and metagenomic sequencing. Another goal is to determine if dental treatments or improved oral hygiene could alter disease activity or slow progression for people living with MS.
"Ultimately, we aim to clarify how the oral–gut–brain inflammatory axis contributes to multiple sclerosis pathophysiology and to explore whether oral health could serve as a novel target for disease modification," said Naito.
The team included researchers from Hiroshima University's Department of Clinical Neuroscience and Therapeutics as well as its Department of General Dentistry.
