The Gut-Brain Axis (GBA): The Missing Link in Depression

There is growing evidence that the trillions of microbes inhabiting our gastrointestinal tracts (commonly referred to as microbiome or gut microbiota) play a mysterious yet significant role in many aspects of our mental health—ranging from psychological resilience to neuropsychiatric disorders

Research shows that the gut microbiota affects your mood, sleep and stress levels.

The term microbiome refers to all microorganisms and their genetic material living in the body, and the term microbiota refers to populations of microorganisms present in the body’s various ecosystems (for example, the gut microbiota and skin microbiota).There are 1014 microorganisms (1014=100,000,000,000,000) in the gut, which is 10-fold greater than the number of human cells.

Various bacterial families–such as firmicutes, bacteroidetes, actinobacteria and proteobacteria–reside in the human intestine. The human gut microbiota contains more than 1,000 species and over 7,000 subspecies. 

Gut bacteria can produce active metabolites for human organ systems.

Lactobacillus and Bifidobacterium synthesize gamma-aminobutyric acid (GABA) from monosodium glutamate. Gamma-aminobutyric acid (GABA)  is the main inhibitory neurotransmitter in the brain

Escherichia coliBacillus and Saccharomyces produce noradrenaline an important neurotransmitter.

CandidaStreptococcusEscherichia and Enterococcus produce serotonin. Serotonin is the neurotransmitter that is dominantly responsible for regulating our sense of calm. Feelings of contentment, satisfaction and well-being are generated by this neurotransmitter, prompting relaxation and peace.

Bacillus and Serratia produce dopamine.  Dopamine is the neurotransmitter that largely controls The pleasure and reward centres of the brain. It helps us to initiate physical movement. It regulates the strength and nature of emotions, enabling the ability to implement plans to obtain recognized rewards.

The gastrointestinal system (GIS) is the largest immune organ in the human body.

The gut epithelium is the largest mucosal surface in the body. In healthy conditions, the tight junctions in the gut epithelium (occludin, adhesion molecule and zonula occludens) and mucus layer form a physical barrier to bacteria and foreign antigens. 

The vagus nerve—which is the longest nerve in the human body and wanders from the brainstem to the lowest viscera of your intestines—is like a communication superhighway of connectivity between your gut and brain

The microbes of the gut microbiota interact with the Gut-Brain-Axis (GBA) through the following pathways.

The Vagus nerve

Neuroendocrine (gut hormones)

Interference with Tryptophan metabolism – Approximately 95% of Serotonin (5-HT) is produced by gut mucosal enterochromaffin cells.

The Immune System – The gut associated lymphoid tissue comprises 70% of the body’s immune system and can be conceptualised as the largest immune organ in the body.

Altered Intestinal Permeability – Chronic stress has been shown to alter intestinal permeability (leaky gut syndrome), which is associated with a low-grade inflammation that can be functionally linked to psychiatric disorders such as depression.

Production of Microbial Metabolites – Many species of Lactobacillus and Bifidobacterium produce gamma-aminobutyric acid (GABA), which is the main inhibitory neurotransmitter in the brain. In addition, Candida, Escherichia, and Enterococcus produce the neurotransmitter serotonin, while some Bacillus species have been shown to produce dopamine. Bacteria also produce short-chain fatty acid (SCFAs), such as butyric acid, propionic acid and acetic acid, that are able to stimulate sympathetic nervous system, mucosal serotonin release and thus influence the memory and learning process in the brain.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4662178/

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