Scientists in Japan have discovered that a common type of bacteria usually found in the mouth and digestive system may be linked to a higher risk of stroke and poorer recovery after a stroke.
The findings suggest that oral health and gut bacteria may play a much larger role in brain health than researchers once believed.
The study focused on a bacterium called Streptococcus anginosus. This bacterium is commonly found in the mouth, throat, and digestive tract. In many people it causes no problems, but researchers found that it appeared much more often in people who had recently suffered a stroke.
The findings will be presented at the American Stroke Association’s International Stroke Conference 2025. Researchers say the study could eventually lead to new ways of predicting stroke risk and improving prevention through oral health and gut health monitoring.
A stroke happens when blood flow to part of the brain is blocked or when a blood vessel in the brain bursts. Without enough oxygen, brain cells quickly become damaged or die.
Stroke is one of the leading causes of death and long-term disability around the world. Many people who survive a stroke are left with problems involving movement, speech, memory, or daily activities.
Doctors already know that high blood pressure, smoking, diabetes, obesity, and high cholesterol increase stroke risk. However, scientists are increasingly learning that bacteria living inside the body may also affect the health of blood vessels and the brain.
The human body contains trillions of bacteria, especially in the gut and mouth. Many of these bacteria are helpful and support digestion, immunity, and overall health. Problems can happen when harmful bacteria become more common than healthy ones. This imbalance may contribute to inflammation and disease throughout the body.
The Japanese research team had already linked another mouth bacterium, Streptococcus mutans, to bleeding in the brain in earlier studies. Streptococcus mutans is best known as one of the main causes of tooth decay.
In the new study, scientists wanted to explore whether other harmful bacteria might also be connected to stroke.
To investigate this, researchers collected saliva and stool samples from 250 adults in Japan with an average age of about 70 years old. Among them, 200 people had experienced a stroke within the previous week.
The remaining 50 adults had no history of stroke and were included as a comparison group during routine health checks.
The scientists analyzed the bacteria found in the participants’ mouths and intestines. They discovered that Streptococcus anginosus appeared much more often in stroke patients than in those without stroke.
Even after taking other stroke risk factors into account, people with this bacterium in their gut had about a 20% higher chance of having a stroke. Researchers also found that stroke patients carrying the bacterium faced a greater risk of dying or developing serious heart and blood vessel problems during the following two years.
The study also identified some bacteria that appeared to protect against stroke. One bacterium called Anaerostipes hadrus was linked to an 18% lower stroke risk, while another called Bacteroides plebeius was linked to a 14% lower risk.
Scientists are still trying to understand exactly how harmful bacteria may increase stroke risk. One possibility is that certain bacteria trigger inflammation in blood vessels, which may make blood clots or vessel damage more likely.
Another theory is that bacteria from the mouth may enter the bloodstream and affect the heart, blood vessels, or brain directly.
The connection between oral health and stroke is especially interesting because both Streptococcus anginosus and Streptococcus mutans are linked to tooth decay and poor dental health. Researchers believe this may mean that improving oral hygiene could help lower stroke risk.
Simple habits such as brushing teeth regularly, reducing sugar intake, flossing, and using toothpaste that fights harmful bacteria may help support healthier bacteria in the mouth. Regular dental care may also become more important for protecting brain and heart health.
The researchers say that in the future, doctors may be able to use quick tests to check for dangerous bacteria in saliva or stool samples. These tests could potentially help identify people at higher risk for stroke before symptoms appear. Scientists also hope that future treatments may be able to target harmful bacteria directly.
However, the researchers caution that the findings are still early. The study involved a relatively small group of participants and only included people in Japan. Gut and mouth bacteria are strongly influenced by diet, lifestyle, culture, and environment, which means the results may not apply equally to people in other countries.
Experts not involved in the study say the findings are still very important because they add to growing evidence that the gut and mouth microbiome may affect many diseases beyond digestion.
Over the past several years, studies have linked gut bacteria to obesity, diabetes, heart disease, depression, Alzheimer’s disease, and immune disorders.
The next step will be larger studies involving more diverse populations. Researchers also want to study people who have not yet had a stroke to better understand whether harmful bacteria contribute to stroke risk before disease develops.
If future research confirms the findings, doctors may eventually include oral and gut bacteria testing as part of routine stroke prevention. This could open the door to completely new ways of reducing stroke risk through dental care, diet changes, probiotics, or treatments that rebalance bacteria inside the body.
For now, the study is another reminder that the health of the mouth, gut, heart, and brain may all be more closely connected than previously thought.
If you care about stroke, please read studies about how to eat to prevent stroke, and diets high in flavonoids could help reduce stroke risk.
For more health information, please see recent studies about how Mediterranean diet could protect your brain health, and wild blueberries can benefit your heart and brain.
Copyright © 2026 Knowridge Science Report. All rights reserved.
