Gut microbiota and cognitive decline: a scoping review of microbial mechanisms and adaptive responses in dementia

Dementia is a progressive disease that results in a loss of mental capacity. Some of the most affected cognitive skills are memory, orientation, and language. These skills are also associated with behavioral shifts such as increased agitation and apathy, worsening the affected person's quality of life. The most common type of dementia is Alzheimer's disease, and it is especially concerning in older adults. Alzheimer's is characterized by the formation of beta-amyloid plaques and neurofibrillary tangles that are made of hyperphosphorylated tau proteins. These plaques and tangles lead to inflammation in the central nervous system, damage to the connections between neurons, and overall degeneration of the nervous system. Newer studies have started to identify the gut microbiome and the gut-brain axis as components critical to the progression of neurodegenerative diseases. Dysbiosis, which is characterized by an imbalance or loss of microbial diversity in the gut, has been attributed to the worsening of neurodegenerative diseases. The gut microbiome has been shown to have a large impact on the brain and how it responds neurochemically. An imbalance in the gut microbiome has also been shown to lead a person to emotional and cognitive dysfunction. It has been shown that in dementia patients, there is also an associated intestinal dysbiosis and increased inflammation systemically and within the brain. Certain gut bacteria stimulate the production of pro- inflammatory cytokines and neuroinflammation, which is a defining characteristic of diseases associated with dementia. This review is focused on three main aspects in which dysbiosis is related to cognitive decline.