A study found that Streptococcus anginosus , a common bacteria in the mouth and gut, was more common in stroke patients and was associated with an increased risk of stroke and death.
Streptococcus anginosus , a bacteria commonly found in the mouth and digestive tract, may be present at high levels in the intestines of people who have had a stroke. This bacteria is associated with a worse prognosis and an increased risk of death, according to a recent preliminary study presented at the American Stroke Association’s 2025 International Stroke Conference.
“In the future, if there is a rapid test to detect harmful bacteria in the mouth and gut, we could use that information to help calculate stroke risk,” said Shuichi Tonomura, MD, senior author of the study and a neurologist at the National Heart and Brain Center in Osaka, Japan. “Targeting these specific harmful bacteria in the mouth could help prevent stroke.”
The role of intestinal and oral bacteria in health
We all have trillions of bacteria in our gut, collectively known as the gut microbiome. There is also another community of bacteria in our mouths called the oral microbiome. Most of these bacteria have a positive effect on the body and support normal bodily processes, such as digestion. However, when beneficial and harmful bacteria are out of balance, diseases can occur, Tonomura notes.
Previously, these researchers found that another cavity-causing bacteria, Streptococcus mutans , was linked to an increased risk of bleeding inside the brain. In this new study, conducted at Japan’s largest stroke center, the researchers quantified all detectable bacteria in both the saliva and gut of people who had recently suffered any type of stroke, comparing them to stroke-free people of similar ages who underwent routine medical checkups.
Researchers found that one species of bacteria, Streptococcus anginosus, was significantly more abundant in the saliva and gut of people who had suffered acute strokes than in a control group of people who had not suffered strokes.
Analysis of different bacterial groups showed:
- Streptococcus anginosus in the gut was independently associated with a 20% higher risk of stroke after controlling for vascular risk factors, while Anaerostipes hadrus (a gut bacteria associated with beneficial effects) was associated with an 18% lower risk and Bacteroides plebeius (a gut bacteria common in the Japanese) was associated with a 14% lower risk.
- During a two-year follow-up period, stroke survivors with Streptococcus anginosus bacteria in their gut had a significantly increased risk of death and major cardiovascular events during the two-year follow-up period.
- No increased risk of mortality or poor outcomes was observed in stroke survivors with Anaerostipes hadrus and Bacteroides plebeius compared with controls.
Dental health and stroke prevention
“Our findings provide new insights into the link between oral bacteria and stroke risk, as well as potential strategies to prevent stroke. Both Streptococcus mutans and Streptococcus anginosus are bacteria that contribute to tooth decay by producing acids that destroy tooth enamel. This highlights the importance of preventing tooth decay, which can be achieved by reducing sugar intake and using toothpastes that target these bacteria. “Maintaining good oral hygiene is essential,” says Tonomura.
Researchers hope to conduct similar studies in people who have never had a stroke but who have risk factors for one.
“Expanding the analysis to a population at risk for stroke is important to understand the broader implications and develop potential prevention strategies for people at risk for stroke,” Tonomura said.
Because the study was conducted in a Japanese population with a relatively small sample size (189 stroke patients and 55 non-stroke controls), these findings might not be fully generalizable to other larger and more diverse populations or groups.
“Gut and oral microbiota are strongly influenced by lifestyle. Other types of bacteria may be important contributors to stroke in other countries,” Tonomura said.
“Poor oral health can lead to inflammation and increase the risk of stroke. “The elevated levels of Streptococcus anginosus found in the gut of patients who have recently had a stroke are very interesting, and the fact that these levels are associated with increased mortality two years after stroke suggests that they may play a role in ongoing stroke risk,” Louise D. McCullough, MD, PHD, FAHA, who is the Roy M. and Phyllis Gough Huffington Endowed Chair in Neurology at McGovern Medical School; chief of neurology at Memorial Hermann Hospital-Texas Medical Center; and co-director of UTHealth Neurosciences, all in Houston, who was not involved in the study, said. “Since this study was conducted after a stroke had occurred, examining a larger group of people who have not had a stroke but have risk factors may help us better understand how these bacteria relate to stroke risk. This knowledge could help improve prevention strategies.” McCullough also co-authored a commentary on stroke and the microbiome published in the January 2024 issue of the American Heart Association’s journal Stroke.
Details of the study, setting or design:
- The study included 250 people (mean age 70 years, 40% women, all Japanese). Two hundred patients diagnosed (by brain imaging) with a stroke (either hemorrhagic or clot-related) in the past seven days were compared with 50 age-matched people with no history of stroke who were undergoing routine health screening. The control group may have other health problems, such as high blood pressure, diabetes, or high cholesterol.
- Stroke patients are examined at the National Heart and Brain Center in Osaka, the largest stroke center in Japan. Control subjects were participants in the ongoing population-based Suita study, which examines cardiovascular risk factors in middle-aged and older adults.
- Researchers analyzed oral and gut microbiota using saliva and stool samples taken from stroke patients and controls between July 2020 and July 2021. During the two-year follow-up period, they looked at how many stroke patients died or experienced new cardiovascular events.
Meeting: ASA International Stroke Conference 2025
