The human microbiome is a unique collection of microorganisms that includes bacteria, fungi, archaea, and protists. Although the size of each microorganism is not visible to the human eye, their total number corresponds to the number of human cells, while the total number of bacterial protein-coding genes is hundreds of times greater than that of human genes. Most microorganisms live in the gut, but bacteria have also been identified in other parts of the body, such as the stomach, mouth, respiratory tract, skin, genital tract, and even the eye.
The invisible presence of the microbiota mediates a number of important processes: from metabolic and immune to cognitive, and its composition deviation is associated with various pathological conditions, such as allergic and autoimmune diseases, diabetes, obesity and cancer. Why are intestinal bacteria important to human health and what functions do they perform?
Lifestyle and nutrition source of the gut bacteria
Of course, one single bacterium in our body cannot significantly affect human health. But bacteria are “social” organisms. By producing specific molecules, they understand how many bacteria of the same or another species are present in the environment. This type of communication is called quorum sensing and allows bacteria to unite and perform the same function at the same time. The influence of an entire community is already felt by the host, and can be beneficial or damaging.
Bacterial composition of the gut largely depends on the diet maintained by the person. In turn, “selected” bacteria affect a person's ability to absorb and metabolize nutrients. The food absorbed by a person mainly consists of nutrients such as proteins, fats, and carbohydrates. While fats, proteins, and simple sugars are easily and almost completely digested by our body in the upper intestines, chemically more complex carbohydrates are difficult to digest or not digested at all by human enzymes. Indigestible carbohydrates, dietary fiber, would have left our body if there were no bacteria living in our lower intestine.
Bacterial metabolic products with positive effect on humans’ health
One of the main nutrition sources for intestinal bacteria is carbohydrates, and their lack causes bacteria to look for alternative ways to obtain energy for their lives. The result of this may be the synthesis of molecules that are toxic for human health. The main products of bacterial fermentation of carbohydrates are short-chain fatty acids (SCFAs) and gases.
The three main SCFAs are acetate, butyrate, and propionate. These fatty acids are involved in various physiological pathways, such as:
- lipid metabolism
- protection from inflammation
In studies with genetically or diet-induced obese mice, a high-fiber diet was shown to reduce body weight, insulin resistance, and food consumption.
Bacteria in our intestines can also metabolize the amino acid tryptophan from food into indoles and its various derivatives. Indoles, naturally found in essential oils derived from jasmine and citrus, are often used in perfumery as an odor fixative, although in our body they activate specific Ah-receptors. These regulate enzymes that promote the breakdown of chemicals alien to living organisms, xenobiotics. Ah-receptors can even signal to the body that a food contains a toxic chemical, thereby causing revulsion to it.
One of the tryptophan metabolism products, indole derivative, indole-3 propionic acid (IPA) improves metabolism, strengthens gut barrier function, increases immune response, and even be an anti-inflammatory agent. IPA also protects pancreatic β-cells that secrete insulin, an important hormone in utilization of glucose in the body. People with higher concentrations of IPA were less likely to develop type 2 diabetes.
Bacterial influence on human cognitive and behavioral features
In addition to local actions and effects on the intestines or nearby organs of the abdominal cavity, it turned out that the metabolites secreted by bacteria. Bacterial products can affect brain function although passing through the blood-brain barrier or changing the concentration of human neurotransmitters. Neurotransmitters are biologically active chemicals through which nerve cells "communicate" among themselves or with muscle or glandular cells.
The so-called brain-gut axis can work in two directions: “top-down” from the brain to the intestine, or “bottom-up” from the intestine to the brain. Intestinal bacteria are generators of such molecules as glutamate, GABA, acetylcholine, dopamine, serotonin, norepinephrine, tyramine, phenylethylamine, tryptamine.
All of them are neurotransmitters, the change in the concentration of which was established in such neurological disorders as:
- Alzheimer’s disease
- Parkinson’s disease
Bacteria - vitamins suppliers
An equally important product of the metabolism of bacteria is their ability to produce vitamins, such as: vitamin K and B group vitamins. Although most of them cater to the needs of the bacteria themselves, some of them can be used by the host as well.
It has been shown that germ-free rats whose food was deficient in vitamin K developed cardiovascular diseases. While dietary deficiency of vitamin K for 3-4 weeks in humans with healthy and diverse microbiota does not lead to avitaminosis. However, people who used broad-spectrum antibiotics had a significant decrease in levels of plasma prothrombin (an essential component of the blood coagulation system in humans and animals).
Bacterial metabolic products with negative effect on humans’ health
With proper nutrition, lack of antibiotic treatment, active lifestyle and absence of chronic diseases, the balance of bacteria and their metabolites is maintained. Bacteria limit each other’s number, preventing a common species from occupying the entire space, and thereby preserving their diversity. Disruption of this balance, as well as dysbiosis (decrease in the number of bacteria), has been associated with a wide range of concerns, from allergies to cancer. Furthermore, not all metabolic products by intestinal bacteria have a positive effect on our health.
One of the metabolic products of intestinal bacteria with negative effect is branched-chain amino acids (BCAAs), a group characterized by a branched side chain structure. Elevated blood levels of BCAAs have been associated with insulin resistance (which can lead to type 2 diabetes and obesity),and can even predict the development of pancreatic cancer and obesity-related cancers in women.
It has also been found that bacteria can metabolize histidine into imidazole propionate, which helps to inhibit the ability of insulin to bind to its receptor, leading to non-uptake of glucose by cells, accumulation of glucose in the blood, and type 2 diabetes.
Human gut bacteria are also able to convert choline and L-carnitine, which are abundant in meat, fish, and eggs, into trimethylamine N-oxide (TMAO). Studies have shown that elevated TMAO levels increase platelet hyper-reactivity and thrombosis, leading to an increased risk of cardiovascular disease in both animals and humans.
Gases such as hydrogen, carbon dioxide and methane are also an inevitable result of bacteria working in an oxygen-free (anaerobic) environment, which is the human intestine. Their formation also affects human health. For instance, hydrogen is involved in the production of hydrogen sulfide, which is highly toxic to colon cells and potentially associated with inflammatory bowel diseases.
Many studies on model organisms and humans have shown that bacteria are not just passive passengers in our body, but perform a wide variety of functions, as explored above. Bacterial composition and their subspecific species are considered as potential biological markers for various diseases. Also, treatment methods using fecal transplantation are developing more rapidly. Further research and development of new technologies should tell us even more about bacteria life in our body.