The microbiome is the population of microorganisms-such as bacteria, fungi, viruses, protozoa, and others-that exist in humans, each of which may have a helpful or a harmful role. Our bodies contain more than 10 times the number of microbial cells than they do human cells. The largest population of microbes exists in the intestinal tract, thought of by some as the largest immune organ in the body. Intriguing research from across neurological diseases has raised the possibility that some microbes in the gut can modify the production of chemicals occurring in the brain.

Cure Alzheimer’s Fund researchers are investigating whether the microbiome may impact the progression of early Alzheimer’s disease pathology.

More than a century ago, the Nobel laureate Elie Metchnikoff postulated that ‘good gut bacteria’ may delay senility and have beneficial effects for the symptoms of anxiety and depression associated with deteriorating cognition. Manipulation of the gut microbiome has since become common practice with widespread consumption of probiotics. While the broad array of health benefits claimed for probiotics has failed to be substantiated in controlled clinical studies, findings have shown that the gut microbiome is essential for normal brain function.

Signal molecules from the microbiome can enter the central nervous system and modulate brain activities, sometimes producing profound effects in animal models. Recent revelations regarding Alzheimer’s disease show that there exists a two-way communication system between the microbiome and brain. Findings suggest that under disease conditions, the “back-and-forth” between the brain and microbiome may become disrupted and reinforce harmful pathways that promote pathology. Abnormal brain activity can shift conditions in the gut and lead to a rise in gut bacteria linked to neuroinflammation and poor health outcomes. Change in the gut microbiome lead, in turn, to increases in metabolites that exacerbate neuroinflammation, anxiety, and depression in the brain. It is not be surprising to learn that brain health is intricately linked to the state of the rest of the body.