Common mouth and gut bacteria may increase stroke risk

A common bacteria usually found in the mouth and gastrointestinal tract appears to be abundant in the guts of people with stroke, and is associated with a worse prognosis and increased risk of death. That’s according to a preliminary study that was presented at the American Heart Association’s International Stroke Conference 2025, held last week in Los Angeles.

Everyone has trillions of bacteria in their gut, collectively known as the gut microbiota. There is also another community of bacteria in the mouth called oral microbiota. Most of these bacteria positively affect the body and assist with normal body processes, such as digestion. However, when the helpful and unhelpful types of bacteria get out of balance, illness can happen.

Previously, researchers have found that a bacteria that causes tooth decay, Streptococcus mutans, was associated with a higher risk of bleeding inside the brain. In this new study, conducted at Japan’s National Cerebral and Cardiovascular Center, researchers quantified all detectible bacteria in both the saliva and guts of 189 people who had recently had any type of stroke, comparing them to 55 people without a stroke of similar age undergoing routine medical check-ups.

The researchers found that one species of bacteria, Streptococcus anginosus, was significantly more abundant in the saliva and gut of people with acute stroke than in the control group of people who had not had a stroke. An analysis of various groups of bacteria found:

• Streptococcus anginosus in the gut was independently associated with 20% higher odds of stroke after controlling for vascular risk factors, while Anaerostipes hadrus (a gut bacteria associated with beneficial effects) was associated with a drop in risk by 18% and Bacteroides plebeius (a gut bacteria common in the Japanese population) was associated with a drop in risk by 14%.
• Stroke survivors with Streptococcus anginosus in the gut had a significantly higher risk of death and major cardiovascular events over the two-year follow-up period.
• Increased risk of death and poor outcomes was not noted in stroke survivors with Anaerostipes hadrus and Bacteroides plebeius compared to controls.
   

“Our findings offer new insights into the connection between oral bacteria and the risk of stroke, as well as potential strategies for stroke prevention,” said Dr Shuichi Tonomura, lead author of the study and a staff physician at the National Cerebral and Cardiovascular Center.

“Both Streptococcus mutans and Streptococcus anginosus are bacteria that contribute to tooth decay by producing acids that break down tooth enamel. This highlights the importance of preventing tooth decay, which can be achieved by reducing sugar intake and using toothpaste that targets these bacteria.”

The researchers hope to conduct similar studies in people who have not experienced a stroke but do have risk factors for stroke. Indeed, Tonomura suggested that testing for harmful bacteria could in the future be used to help calculate stroke risk.

Tonomura did acknowledge however that the findings may not be fully generalisable to non-Japanese populations or larger, more diverse groups. “The oral and gut microbiome is strongly affected by lifestyle,” he said. “Other bacteria may be main players in stroke in other countries.”

Image ©American Heart Association