A study presented at ISC 2025 shows elevated levels of Streptococcus (S.) anginosus ? a type of bacteria present in the normal oral and gastrointestinal microflora ? in the mouth and gut of recent stroke survivors in Japan.

The researchers evaluated the salivary and gut microbiomes of 222 Japanese individuals (average age 73 years). Of these, 169 had a stroke within the past 7 days, while 53 had no stroke history. [ISC 2025, abstract 106]

S. anginosus was significantly more abundant in the salivary (median 0.01 vs 0.00; p=0.02) and faecal samples (median 0.09 vs 0.00; p<0.001) of stroke vs non-stroke patients.

After adjusting for vascular risk factors, S. anginosus (odds ratio [OR], 1.20; p<0.01), Anaerostipes (A.) hadrus (OR, 0.82; p<0.01), and Bacteroides (B.) plebeius (OR, 0.86; p=0.01) in the gut were determined as independent stroke predictors.

These imply that S. anginosus in the gut was independently associated with a 20-percent higher risk of stroke, whereas A. hadrus and B. plebeius were tied to reductions in stroke risk by 18 percent and 14 percent, respectively.

This was supported by longitudinal data linking increased mortality and major adverse cardiovascular events (MACE) to S. anginosus in the gut (p_log-rank=0.04) but not to A. hadrus (p_log-rank=0.45) and B. plebeius (p_log-rank=0.19).

Good oral hygiene is key

Lead study author Dr Shuichi Tonomura from the National Cerebral and Cardiovascular Center, Osaka, Japan, noted that the results offer new insights into the link between the oral microbiome and stroke risk.

“[S. anginosus] contributes 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,” he said.

“Poor oral health can lead to inflammation and may increase stroke risk. The increased levels of S. anginosus found in the gut of patients with recent stroke is intriguing, and the fact that levels were linked to increased mortality 2 years after stroke suggests that it may play a role in ongoing stroke risk,” commented AHA volunteer expert Dr Louise McCullough from the UTHealth Neurosciences, Houston, Texas, US, who was not involved in the study.

Next steps

“[Overall,] S. anginosus in the gut was identified as an oral pathobiont in gut dysbiosis, is a central hub susceptible to acute stroke, independently associated with acute stroke, and a predictor of MACE after stroke,” said Tonomura. He added that it could serve as a novel biomarker for predicting stroke and poor outcomes following stroke, underlining its potential as a target for therapeutic intervention.

“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,” he noted.

However, generalizability of the findings may be limited, as the study was confined to a relatively small Japanese population.

The team aims to continue examining the gut microbiome to identify stroke risk, noted Tonomura. “[We can] use the insights to design personalized dietary interventions by tailoring nutrition to each individual’s microbiome. We aim to maximize the protective impact of daily lifestyle habits on stroke risk.”

Another goal is to conduct similar investigations in individuals who have not had 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,” he said.

McCullough mirrored Tonomura’s sentiments, stating that “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.”