"In the future, if there was a quick test to detect harmful bacteria in the mouth and gut, we could use the information to help calculate stroke risk. Targeting these specific harmful oral bacteria may help prevent stroke," said Shuichi Tonomura, M.D., lead author of the study and staff physician in the neurology department at the National Cerebral and Cardiovascular Center in Osaka, Japan.

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, Tonomura noted.

Previously, these researchers found that a different bacteria that causes tooth decay, Streptococcus mutans, was associated with a higher risk of bleeding inside the brain. In this new study, conducted at the largest stroke centre in Japan, researchers quantified all detectible bacteria in both the saliva and guts of people who had recently had any type of stroke, comparing them to people without a stroke of similar age undergoing routine medical checkups.

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:

"Our findings offer new insights into the connection between oral bacteria and the risk of stroke, as well as potential strategies for stroke prevention. 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. Maintaining good oral hygiene is essential," Tonomura said.

The researchers hope to conduct similar studies in people who have not experienced a stroke but have risk factors for stroke.

"Extending the analysis to populations at risk of stroke is crucial for understanding broader implications and developing potential preventive strategies for those susceptible to 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 participants), the findings may not be fully generalisable to other populations or larger, more diverse groups.

"The oral and gut microbiome is strongly affected by lifestyle. Other bacteria may be main players in stroke in other countries," Tonomura said.

"Poor oral health can lead to inflammation and may increase the risk of stroke. The increased levels of Streptococcus anginosus found in the gut of patients with recent stroke is intriguing, and the fact that levels were linked to increased mortality two years after stroke suggests that it may play a role in ongoing stroke risk," said Louise D. McCullough, M.D., P.H.D., FAHA, who is the Roy M. and Phyllis Gough Huffington Distinguished Chair of 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. "Since this study was conducted after a stroke occurred, examining a larger group of people who have not yet had a stroke but do have risk factors could enhance our understanding of how these bacteria are related to stroke risk. This knowledge could help improve prevention strategies." McCullough also coauthored the commentary Stroke and the Microbiome published in the Jan. 2024 issue of the American Heart Association Stroke journal.