2017-10-26 20:10:55 UTC

How Gut Bacteria Impact the Brain

Oct. 26, 2017

Recent trials have demonstrated that specific probiotics can alter brain connectivity in healthy individuals. Dr. Premysl Bercik shares evidence of the gut microbiota's role in the gut-brain axis.

By Premysl Bercik, MD, McMaster University; scientific advisory board member for the AGA Center for Gut Microbiome Research and Education

It is well established that gut bacteria play a key role in shaping the immune system and host metabolism. During the last decade, it has become apparent that the microbiota also affects distant organs, such as the brain, and modulates host behavior. Although the bulk of evidence arises from animal studies, clinical evidence is mounting.

Gut microbiota is an integral part of the gut-brain axis, the bidirectional communication between the digestive system and the brain. Animal studies have shown that germ-free mice (i.e., mice born in sterile facilities free of microorganisms) have altered behavior and changes in brain structure and chemistry compared to conventionally raised mice. Also, modifying gut microbiota composition by antimicrobials alters mouse behavior.

Gut bacteria can communicate with the neural system by multiple pathways, including direct interaction with the immune system. Bacteria can affect production of serotonin in colonic enterochromaffin cells, modulate levels of endogenous catecholamines in the gut lumen and produce  gamma-Aminobutyric acid (GABA) or other neuroactive molecules, including short chain fatty acids (SCFA). The brain can detect the presence of bacterial pathogens through vagal pathways within several hours, before any significant immune reaction is mounted. The effect is bidirectional as sympathetic system activation can in turn, affect the composition of gut microbiota and change its functional properties such as adherence to the gut mucosa.

Gut microbiota profile is altered in patients with chronic GI diseases and psychiatric comorbidities, as well as in patients with primary neuropsychiatric disorders. Recent trials have demonstrated that administration of specific probiotics can alter brain connectivity in healthy individuals, as well as improve depression scores and neural activity in multiple brain areas associated with mood control in patients with irritable bowel syndrome (IBS) and comorbid depression.

Overall, gut bacteria appear to play a significant role in host behavior and brain function. Further understanding of the microbiota-gut-brain axis will likely aid in the management of patients with chronic GI diseases and neuropsychiatric disorders.

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This article is part of the Oct. 27 AGA Microbiome Update, which was emailed to all AGA members.

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