Oral microbiome signatures of ageing and cognitive function across distinct geographical cohorts

Declining cognitive function in older adults may be linked to reduced nitric oxide (NO) bioavailability, which is supported by oral nitrate-reducing bacteria. Oral microbiome community structure varies geographically, complicating understanding of microbiome-cognition relationships across populations. This study aimed to identify consistent associations between the oral microbiome and cognitive function across two geographically distinct cohorts from the UK and Belgium, and to explore age-related differences in NO biomarkers, cognitive performance, and microbiome composition using 16S rRNA sequencing. Sixty healthy participants (15 young and 15 older adults per location) completed a cognitive test battery, blood pressure measurements, plasma and saliva collection for nitrate and nitrite analysis, and a tongue swab for oral microbiome assessment. Despite geographical differences in oral microbiome composition (PERMANOVA P = 0.04), age-related microbial shifts were consistent across geographic cohorts, including enrichment of Treponema and Tannerella in older adults. Nitrate-reducing taxa such as Haemophilus parainfluenzae and Neisseria flavescens were consistently associated with cognitive performance measures in both cohorts ( P < 0.05). In the UK, higher plasma [nitrate] was associated with improved Digit Symbol Substitution Test (DSST) accuracy (β = 19.2, P < 0.01) and reaction times (β = −3444 ms, P < 0.05), but poorer Emotion Recognition Task (ERT) accuracy (β = −5.219, P < 0.05), while UK plasma [nitrite] was associated with slower Balloon Analog Risk Task (BART; β = 2277 ms, P < 0.001) and ERT (β = 8577 ms, P < 0.05) reaction times. In Belgium, saliva [nitrate] was associated with slower Visual Object Learning Test (VOLT) reaction times (β = 202 ms, P < 0.05), while in the UK, saliva [nitrite] correlated with slower BART (β = 111 ms, P < 0.05) and LOT (β = 1888 ms, P < 0.01). Overall, geography emerged as a major driver of oral microbiome variation, yet consistent age-related microbial and cognitive associations were preserved across populations. These findings emphasise the need to account for geographic context in microbiome-health research. A better understanding of oral microbial patterns may help inform future mechanistic or interventional studies to promote cognitive health in ageing.