Tongue bacterium tracks with faster responses on some tests in 60 UK and Belgian adults

Researchers looked at tongue bacteria because of a chemical route. Some nitrate from foods such as beetroot enters the bloodstream and comes back into the mouth. There, tongue bacteria help convert it to nitrite, which can then contribute to nitric oxide, a molecule that helps blood vessels widen ^[19]. That route is one reason researchers are asking whether oral microbes are connected to memory, attention and processing speed ^[19]. In a 2025 PNAS Nexus paper, the same group looked at mouth bacteria and markers of nitric oxide in older people, among them people with mild cognitive impairment ^[17].

The PLOS ONE study is small and evenly divided: two countries, two age bands, four groups of 15 ^[4][5][21]. The younger volunteers were mostly in their twenties. The older ones averaged in their sixties or seventies ^[7]. Everyone was a non-smoker, and people with recent antibiotic use or a history of oral disease were excluded ^[9]. All had passed cognitive screening, so the score differences are variation among healthy people ^[20]. Each person sat tests of memory, attention, emotion recognition and processing speed, and gave blood, saliva and tongue swabs ^[8].

The cross-country result is looser than it first sounds. People with more Prevotella histicola responded faster in both countries, but on different tests in the UK and in Belgium ^[10]. Haemophilus parainfluenzae went in opposite directions on emotion recognition in the two countries ^[11].

In our view the chemistry is too mixed to support the nitric oxide explanation. The nitrate reducer that was more common in younger mouths was also more common in the lower-scoring UK group ^[15]. In the UK, higher plasma nitrate went with better performance on one test but poorer emotion-recognition accuracy. Higher nitrite went only with slower responses ^[18].

Age also changed the microbes. Seventeen species differed in abundance between younger and older adults ^[13]. The genera Treponema and Tannerella, which include species associated with gum disease, were more common in older mouths ^[14]. Bacterial communities also differed between the UK and Belgian participants ^[16]. A bacterium that rises or falls with age will correlate with test scores whether or not it does anything to the brain. We would want to see results that separate age and country from each species before reading any single bacterium as more than a marker.

Diet was not controlled, and physical activity and sleep were not comprehensively assessed. ScienceAlert noted that these could influence the bacteria ^[3]. Diet is also where the nitrate comes from ^[19]. The thing this doesn't tell you is whether changing someone's oral bacteria would change how fast they think. The study did not test that, and it did not show that the nitrate process caused any difference in scores ^[1].

The study set many bacterial species against several kinds of test, with 30 participants per country. In a design like that we'd expect some associations to turn up by chance. The authors say larger studies are needed to separate genuine biological patterns from statistical variability ^[2].