This review describes the complex interplay between the gut microbiome gut microbiome, brain and enteric nervous system – also called “brain-gut-microbiota axis” known as the “brain-gut-microbiota axis”. The microorganisms in the gut (microbiota) not only influence digestion, but also the immune system, the barrier function of the gut, the perception of stress and even mood, cognition and behavior.
In animal models, germ-free mice show clear differences in brain structure, stress reactions and behavior. Studies show that intestinal bacteria communicate directly with the brain via the vagus nerve, the immune system or metabolic products such as tryptophan. A disturbed microbiota (dysbiosis) is associated with irritable bowel syndrome (IBS), depression, autism, obesity and inflammatory bowel disease (IBD).
Conclusion: The targeted influencing of the microbiome (e.g. through probiotics) could play an important role in the treatment of gastrointestinal, neurological and mental illnesses in the future.
Introduction
The term “brain-gut-microbiota axis” refers to a complex communication system between the brain, gut and gut microbiota. It comprises the central nervous system (CNS), the enteric nervous system (ENS), the neuroendocrine system, the immune system and the intestinal flora. Bidirectional communication takes place via nerve pathways (e.g. vagus nerve), hormones and cytokines.
Development and function of the microbiota
Intestinal colonization begins at birth and develops into a stable “adult” microbiota by the first year of life. Influencing factors such as birth mode, diet, infections or antibiotics can disrupt the balance. An intact microbiota is essential for:
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Barrier function of the intestine
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Regulation of motility
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Metabolic processes
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Modulation of the immune system
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Protection against pathogens
Germ-free animals show, for example, reduced immune responses, enlarged appendix and reduced brain maturation.
Influence on the brain and behavior
The microbiome directly influences the CNS, in particular via the hypothalamic-pituitary-adrenal (HPA) axis. In germ-free mice it comes to:
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Exaggerated stress reaction (increased cortisol, ACTH)
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Reduced neurotransmitter production (e.g. serotonin, GABA)
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Changes in behavior (e.g. anxiety, memory)
Probiotic bacteria such as Lactobacillus rhamnosus or Bifidobacterium infantis can have anxiety- and stress-reducing effects – partly mediated via the vagus nerve.
Microbiota and diseases
Irritable bowel syndrome (IBS):
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Often associatedwith altered intestinal flora, low-grade inflammation and increased permeability.
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Post-infectious IBS occurs in up to 36% of cases following bacterial intestinal infections.
Chronic inflammatory bowel disease (IBD):
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Characterized by altered microbiota (more proteobacteria, fewer firmicutes).
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It is unclear whether this is a cause or a consequence of the inflammation.
Autism:
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Children with autism often show intestinal complaints and altered clostridial composition.
Obesity:
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GF mice are resistant to obesity.
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Microbiota influences energy intake and fat metabolism.
Therapeutic perspectives
Targeted modulation of the microbiome (e.g. with probiotics, prebiotics, antibiotics) is considered a promising strategy, especially for functional gastrointestinal disorders such as IBS.
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Lactobacillus plantarum, B. infantis and others showed positive effects on pain, inflammation and barrier stability.
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Initial studies also point to possible benefits in the treatment of depression.
Conclusion
Research into the brain-gut-microbiome axis has shown that gut bacteria are much more than “digestive helpers”. They play a central role in the health of the entire body – including the brain.
In the future, customized microbiome therapies could become part of the treatment for irritable bowel syndrome, depression, autism or even neurodegenerative diseases. Many mechanisms are still unclear, but the data is promising. The biggest challenge remains to define a clear picture of a “healthy” microbiota and to develop stable, individualized therapeutic approaches