A scientific study has linked the abundance of specific types of bacteria found in the mouth to faster responses in some cognitive tests, but it did not show that these microbes directly improve memory or thinking. Published in the journal PLOS ONE, the study involved 60 healthy adults in the United Kingdom and Belgium and examined the relationship between oral microbes and cognitive performance across different age groups.
The study was led by Joanna E. Lough of the University of Exeter, in collaboration with researchers from the Vrije Universiteit Brussel. The team collected blood and saliva samples and tongue swabs from the participants, who underwent tests measuring memory, attention, information-processing speed and the ability to recognise emotions, as well as blood pressure measurements.
Oral bacteria linked to response speed
The results showed that higher levels of some types of bacteria were associated with faster responses in certain tests. The researchers recorded an association between increased levels of Prevotella histicola and faster responses in some tests among participants in both countries, despite differences between the tasks carried out in the United Kingdom and those used in Belgium.
Haemophilus parainfluenzae was also associated with varying results depending on the test and country. In Belgium, its abundance was associated with faster responses in emotion-recognition and visual-learning tests, as well as greater accuracy in working-memory tests.
Among participants in the United Kingdom, however, it was associated with faster responses in some tasks but with slower responses in emotion-recognition tests. The researchers stressed that a greater abundance of a particular type of bacteria does not necessarily mean an overall improvement in cognitive abilities, given that the observed associations differed according to the tests and countries.
Age explains part of cognitive performance
Age was the most consistent factor in its relationship with cognitive performance, with advancing age associated with lower overall scores and slower responses among participants in both countries. The team recorded differences between younger and older participants in the abundance of 17 types of bacteria.
Bacteria from the genera Treponema and Tannerella, which include species associated with gum disease, were more abundant among the older group. By contrast, one type of nitrate-reducing bacteria was more abundant among younger participants. However, a greater abundance of nitrate-reducing bacteria was not always associated with better cognitive performance.
The same species was found at higher levels among participants in the United Kingdom who recorded lower overall test scores, reflecting the inconsistent relationship between the composition of oral microbes and performance levels across different tasks. The findings build on previous research examining the link between oral microbes and nitric oxide biomarkers in older adults, including people with mild cognitive impairment.
The role of nitrates in the nitric oxide pathway
This relationship has attracted scientific interest because some oral bacteria help convert nitrates found in foods such as beetroot into nitrite, which can contribute to the production of nitric oxide, a molecule that helps blood vessels dilate.
However, the study did not show that this process directly affects cognitive abilities, nor did it test whether memory or thinking speed could be improved by changing the composition of oral bacteria. The researchers described the findings as exploratory because of the limited sample, which consisted of 30 people in each country, and stressed the need for larger studies to verify the associations recorded.
The study did not control the participants’ diets, nor did it comprehensively assess their physical activity and sleep, although these factors may affect both the composition of bacteria in the mouth and cognitive performance at the same time.
