Gut microbiota characterization in ageing, mild cognitive impairment, and Alzheimer’s disease in the context of mediterranean lifestyle in a Spanish population

Alzheimer’s disease (AD) is a neurodegenerative disorder often preceded by a prodromal stage of Mild Cognitive Impairment (MCI). Previous research suggests that gut microbiota (GMB) dysbiosis may contribute to cognitive decline via the microbiota-gut-brain axis (MGBA). Notably, GMB composition patterns can vary across populations and stages of dementia. This study aimed to characterize the GMB in a cohort of older adults from Tarragona (Spain) diagnosed with AD or MCI, or presenting a healthy cognitive status (HC), all of whom follow a Mediterranean lifestyle (ML). The present cross-sectional, multicenter case–control study analyzed fecal samples from 99 individuals,including 31 with AD, 30 with MCI, and 38 HC,aged 60–85 years, recruited from seven hospitals and specialized cognitive centers in the province of Tarragona, Spain. Shotgun metagenomic sequencing was conducted with taxonomic profiling using Kraken2. APOE genotyping was performed from fecal DNA using TaqMan assays. Richness, alpha and beta diversity, differential abundance, multivariate linear modeling, and Jonckheere–Terpstra trend tests were conducted to identify GMB species signatures associated with MCI and AD. Richness, alpha and beta diversity did not differ across groups. Differential abundance analysis identified 109 taxa, of which ten microbial species were shared across comparisons. Notably, several species, including Coprococcus comes and Odoribacter splanchnicus, emerged as replicable candidates, showing both discriminatory value and severity-related declines, alongside taxa with context-dependent or adverse associations. Overall GMB diversity did not differ across cognitive groups, but specific taxa, particularly short-chain fatty acid producers, showed consistent associations with cognitive decline in this ML cohort. These findings support a role for the GMB in AD pathology and suggest that targeting key microbial species may provide novel avenues for prevention and intervention.