Our bodies are covered in microbes – both on the inside and the outside. These microbes change depending on the niche of the body and the local conditions.
Generally, commensal microbial communities fend off incoming pathogens, improving the health of the local body niche. When the number and composition of beneficial commensal microbes are disrupted, the resulting dysbiosis can have serious effects on our health.
Commensal microbial communities in and on our bodies, fight pathogenic intruders and keep us healthy.
For example, the oral microbiome fights pathogens like Porphyromonas gingivalis. These can otherwise cause periodontitis – a disease that destroys the tissue around your teeth. Similarly, a dysbiosis of the vaginal microbiome can result in the growth of vaginosis-causing microbes.
However, depending on the site of residence, local microbiomes can have additional functions. Most importantly and best characterised by, is the gut microbiome and its support in food digestion. This local microbial community metabolises otherwise unbreakable substrates and produces molecules that are important for our metabolism.
Research shows how the gut microbiome and its products also affect our mental health. As such, dysbiosis of the gut microbiome can even result in psychological disorders like anxiety or depression.
Communication between the microbiome and the brain
The microbiota–gut–brain (MGB) axis represents the bidirectional communication between the brain and the gut microbiota. Many factors influence this interaction: the immune system and its chemokines and cytokines; metabolic pathways and metabolic products as well as the central nervous system and its stress hormones and neurotransmitters.
Different bacterial species can metabolise these hormones, metabolites and neurotransmitters in the gut. Additionally, stress and emotions affect the secretion of gastric acid, bile and mucus. Together, these factors alter the environment for microbes and thus the composition of the gut microbiome.
On the other side of the MGB axis, gut microbes produce metabolically active compounds like short-chain fatty acids. These cross the epithelial cell layer and modulate gene expression, neurotransmitter signalling and metabolism. Thus, a change in the microbiome composition can influence our emotions, stress and behaviour.
A change in the composition of the gut microbiome can influence our emotions and behaviour.
Your behaviour shapes your gut microbiome shapes your mental health
Many factors like diet, physical exercise, drug treatment or geographical location impact the gut microbiome composition. By shaping the gut microbiome and thus the produced metabolites, these factors also influence our mental health status and behaviour.
For example, studies investigated the effects of foods that contain probiotic strains like Bifidobacterium infantis, B. longum, Lactobacillus helveticus ROO52, L. rhamnosus JB-1 or L. casei strain Shirota. These probiotic foods were shown to reduce depressive- and anxiety-like behaviour, memory dysfunction and even physical symptoms during stressful periods.
Also, prebiotics like fructooligosaccharide and galactooligosaccharide have similar impacts on our mental health and cognitive behaviour. Researchers claim that prebiotics increase the relative abundance of beneficial microorganisms and thus shape the MGB axis.
We can influence our gut microbiome and mental health with more than food. For example, physical exercise not only helps the immune system and metabolism. It also leads to a richer gut microbiome that helps metabolise lactic acid from the muscle to produce more short-chain fatty acids.
On the contrary, increased alcohol intake or chemotherapy can have opposite effects on our mental health. These compounds harm the epithelial layer of the gut resulting in gut leakiness and inhibited microbial adherence. This changes the gut microbiota and can lead to an increased risk for psychiatric disorders, depression and anxiety.