The Microbiome-Gut-Brain Axis in Neurocognitive Development and Decline

Since the beginning of life on earth, microorganisms have played a significant role in evolution. Throughout the history of Homo sapiens and its precursor humanoid forms, microorganisms have been present at birth and proliferated until death. It is at these extremes of life that the microbiome, especially that within the gastrointestinal tract, is most dynamic and perhaps has its greatest influence on host health. Here, we focus on the role of the gut microbiome as a regulator of brain and behaviour through key points in the human lifespan. We first describe trajectories of the microbiome in early life and ageing, before providing evidence for the existence of sensitive periods in the microbiome-gut-brain axis at these extremes of the lifespan. Finally, we briefly examine potential mechanisms for interactions between the microbiome and the brain during development and ageing.