Microbiota-gut-brain axis and probiotics: potential therapeutic strategies for treating Alzheimer's disease.

The gut-brain axis explains that changes in the intestinal microbiota influence Alzheimer's disease (AD). Short-chain fatty acids produced by the gut microbiome regulate the permeability of the gut and blood-brain barrier. Furthermore, they upregulate brain-derived neurotrophic factor, promote angiogenesis and neurogenesis, and control tau and Aβ proteins, microglial activity, apoptosis, oxidative damage, M1/M2 polarization of microglia, and neuroinflammation, which eventually improves cognitive impairment. This effect is mediated by modification of serotonin, dopamine, and γ-aminobutyric acid levels. Compared to healthy controls, mild cognitive impairment and AD were associated with low levels of Firmicutes and Bifidobacterium and high levels of Proteobacteria and Bacteroidetes. Lactobacillus and Bifidobacterium species were effective in improving cognitive function. More longitudinal research is needed to investigate precision medicine in patients with dysbiosis in the preclinical stages of the disease. This review describes the role of the gut microbiome and probiotics in AD.