Microbiota-Gut-Brain Axis in Neurological Disorders

The human body accommodates a multitude of microorganisms that inhabit various anatomical sites, such as skin, mucosa, gastrointestinal tract, respiratory tract, urogenital tract and mammary glands, and are collectively defined as the microbiome, the composition and functions of which are crucial to health and survival (Moos 2016). Among the different organ systems, that coordinate the functions of the human body, nervous system, comprising the brain and spinal cord, plays a primary role in controlling awareness, movements, sensations, thoughts, speech and memory by integrating sensory information and responding accordingly. Furthermore, the microbiome engages in complex interactions with the organ systems of the human body thereby regulating the functions of both the entities. Such relationships between the nervous system (CNS) and the gut microbiome have been explored in detail and is termed as microbiome-gut-brain (MGB) axis, a complex bidirectional inter-communication that exists between the gut microbiome and the crucial areas of the CNS (Malan-Muller et al. 2018; Cryan 2019). Various neuroactive compounds such as neurotransmitters, metabolites, cytokines, and hormones are synthesised by the gut microbiota and the host as a result of this interaction. These neuromodulatory substances gain access to the brain by different pathways, thus affecting the local homeostasis. Dysregulation of the MGB axis has been implicated in the pathogenesis of various neuroinflammatory, neurodevelopmental and neurodegenerative diseases which is mediated by either a direct line of communication through vagus nerve or immune system activation or both (Cryan 2019). In turn, gut microbiota composition is influenced by the brain in response to stress and endocrine factors (Tremlett et al. 2017).