Microbiome role in the gut-brain axis, part II

But what exactly do these bacteria, fungi and so on do? Do they affect more aspects of our lives than we think they do? Take a look at a simple illustration of the bidirectional communication pathways between the gut microbiota and the brain. We saw a range of molecules originating in the gut that are involved in the upstream part of the communication system.

The microbiota produces: - Short-chain fatty acids (SCFA), which affect lipid, glucose and cholesterol metabolism in various tissues, as well as regulate T-reg cells induction in the colon and control satiety hormones. ‐Neuropeptides and neurotransmitters - The microbiota is associated with the synthesis of neuroactive molecules like serotonin, which is responsible for anxiety, happiness and mood. The microbiota is involved in the “fight or flight” response driven by the activation of the sympathetic nervous system. We nearly understand how exposure to that response plays an important role in the deregulation of the intestinal ecosystem. We divide microbiota into potentially harmful bacteria like Clostridia, which produce enterotoxins; Staphylococcus, Proteus, Pseudomonas aeruginosa, which produce toxins.

And potentially good bacteria like Bifidobacterium and Lactobacillus, which help in digestion and have antitumor activity; Eubacterium, Fusobacterium, which produce short­chain fatty acids, and Campylobacter jejune - a potential tumor marker. Dysbiosis of gut bacteria has been implicated

in: intestinal diseases like colorectal cancer and inflammatory bowel disease; systemic diseases like diabetes, metabolic syndrome, kidney toxicity, atopy, type 2 diabetes and hypertension; and neurological diseases like Parkinson’s, Alzheimer’s, multiple sclerosis or autism. The microbiome acts as an organ. It maintains homeostasis. It contributes to the protection from pathogens, hosts fat storage, regulates brain physiology and behavior, as well as regulates food digestion and metabolism rate. This “job” is dependent of food quality. A healthy diet lets microorganisms exist in the eubiosis state. Increased number of Bacteroidetes and Firmicutes increase microbial diversity and concentration of short-chain fatty acids. Normal levels of neurotransmitters are observed, the same as the physiological immune response. The intestine has normal tight junctions.

There is a healthy level of inflammatory cells in circulation. In summary, a healthy Central Nervous System functioning is observed. How a high-fat diet changes microbiome response? A decrease in the number of Bacteroidetes and Firmicutes is observed, and a decrease in microbial diversity. Altered number of neurotransmitters is also observed, which causes the increase of anxiety, depression, sensitivity to stress, and a decreased learning and memorizing performance. Leaky gut causes an inflammatory response, which influences neuro-inflammation and dysregulates Central Nervous System functioning. Chronic infection with Helicobacter pylori, responsible for stomach ulcers, induces anxiety-like behavior in mice and reduces food intake. Definitely, the microbiome influences the gut-brain axis. Bacteria produce enzymes that help to digest substrates from food, especially saccharides.

Hehemann’s group found the new glycoside hydrolase - the enzyme which helps to digest porphyrin in human stool as a gene of Bacteroides plebeius. The data showed that it is present only among the Japanese population, not that of the USA. It suggests that microorganisms have “tools” to help digest some substrates and to modify the human genome. Gut bacteria are the main source of vitamin K and some B-complex. They have a cognitive function. Bacteria give a human the opportunity to receive some micronutrients and phytonutrients from plants. Bacteria have an impact on our immune system, as they enrich immune cells, mucus and immunoglobulin A.

It was found that a high-fat diet and a high-sugar diet shift the immune response into inflammation, and decrease the number of CD4 cells, which help to keep homeostasis. To conclude, the human microbiome is the place where our body operates with the outside world. The microbiome acts as the frontline of our immune system, as it is constantly exposed to new microbes and compounds which come with the food.

In summary let’s cite Hippocrates: ”Let food be thy medicine and medicine be thy food”. And Jean Brillat-Savarin “Tell me what you eat and I will tell you what you are”.