The Human Microbiome Project (HMP) is a project initiated in 2008 to determine the characteristics of the factors affecting the distribution and evolution of microorganisms that make up the human microbiome and microbiota. Thanks to this project, more than 10,000 bacteria and fungi and more than 3000 virus species have been identified in humans until today, and the importance of microbiota in human life has been understood. Intestinal microbiota consists of microorganisms that perform various beneficial functions in the digestive system. There are 1013 cells in an average human body. The number of cells in the intestinal microbiota is ten times this number. Approximately 10% of the human body consists of human cells and 90% of it consists of cells located in the microbiota.
Microbiota has an important role in humans, diseases and health conditions. The digestive system contains most of the microbiota due to its very large surface area and rich nutrients for microorganisms. Most of these bacteria that make up the microbiota are found in the large intestine and contain bacteria that make up 60% of the dry fecal weight. The number of species living in the digestive tract is between 300 and 1000. It is stated in most sources that the number of species is around 500. However, it is thought that almost 99% of bacteria belong to only 30-40 species. In other words, while there are many species in the intestinal microbiota, certain species predominate in this bacterial community.
The relationship between the intestinal microbiota and humans is symbiotic, that is, both parties benefit from this relationship. However, some intestinal microorganisms can also cause diseases when they find an opportunity. Although people can survive with no intestinal microbiota, intestinal microbiota has some vital functions. These functions can be summarized as follows:
• To convert unused substances into usable substances by fermentation
• To train immune system
• To prevent the growth of harmful organisms
Microorganisms are less common in the upper gastrointestinal system (esophagus, stomach) and small intestine. The large intestine contains the largest number of bacteria and the activities of these bacteria make the large intestine the most metabolically active organ in the body. In the first part of the large intestine, carbohydrates are fermented, and then protein and amino acids are broken down. Different species of bacteria are found in different parts of the intestine due to the intestinal pH, immune system and bowel movements.
Not all types of bacteria in the intestines can be identified. Since the culture (in vitro reproduction) of some species is not possible today. Although the number of species in the intestinal microbiota varies greatly from person to person, it remains quite constant for a given person over time. Intestinal microbiota transforms into adult microbiota in the first 3 years of life and tends to remain constant.
Functions of the intestinal microbiota
Intestinal microbiota has many beneficial functions for humans. These particularly include helping the breakdown and absorption, that is digestion, of the foods, and ensuring a balanced response from the immune system.
Carbohydrate fermentation and absorption
In absence of intestinal microbiota, the human body cannot digest and use part of the carbohydrates, because the enzymes necessary for the digestion of polysaccharides are only produced by some intestinal bacteria. It has been demonstrated that rodents that grow in a sterile environment and have no intestinal microbiota had to eat 30% more to remain at the same weight as compared to normal animals. Compounds that cannot be completely digested without the help of bacteria include some types of carbohydrates (such as starch), oligosaccharides, and lactose. In addition to these, the proteins of the intestinal mucosa and exfoliated intestinal epithelial cells cannot be completely digested without the help of bacteria.
Bacteria convert the carbohydrates they ferment into Short-Chain Fatty Acids (SCFA). These are used by host cells and create an important energy source in humans. In addition, these fatty acids increase the water absorption capacity of the intestine and reduce the number of some harmful bacteria. SCFAs enable the growth of both intestinal cells and beneficial bacteria. Important SCFAs include acetic acids, propionic acids and butyric acids. Fermentation also results in the formation of organic acids such as lactic acids and gases. These organic acids are used by the body for energy production and some important functions.
Another type of fermentation is proteolytic fermentation, that is, fermentation of proteins. With proteolytic fermentation, proteins such as collagen and elastin in dead host and bacterial cells are broken down. This fermentation also results in the formation of some SCFAs.
Bacteria also produce vitamin K2 and ensure it to be absorbed by the body. In addition, SCFAs produced by bacteria help the absorption of calcium, magnesium and iron from the intestines.
Impacts on the immune system
The intestinal microbiota has a continuous impact on the intestinal and systemic immune system of the host. The microbiota plays a key role in both the early development and the lifelong functioning of the immune system in the intestinal mucosa.
As soon as an infant is born, bacteria start to locate in the digestive system. The bacteria that locate first have an impact on the immune system and enable themselves to be identified as belonging to the host. Thus, the first bacteria determine the content of a person's microbiota throughout his/her life. Therefore, the positive impacts of normal delivery and breast milk on infants are very important.
Impacts of antibiotics on microbiota
Reducing the number of bacteria with the use of broad-spectrum antibiotics has an impact on the health of the host and its ability to digest foods. The intestinal microbiota may be damaged when antibiotics are taken to treat bacterial diseases or when the meat of antibiotic-administered animals is unknowingly eaten. This may occur by having a direct impact on the intestinal microbiota or by allowing pathogenic bacteria to proliferate. Another negative impact of broad-spectrum antibiotics is the increased number of antibiotic-resistant bacteria.
Changing the number and species of bacteria in the intestinal microbiota may cause impaired balance of the digestive system. Another impact of the decreased native good bacteria is the proliferation of harmful bacteria.
Impact of nutrition on microbiota
Diet also plays an important role on microbiota. Foods taken with diet are broken down by enzymes and absorbed from the intestine. However, the intestinal microbiota has a key role in the metabolism of fibers that cannot be broken down by enzymes in the diet. In many studies, the differences in microbiota have been demonstrated with the classification analysis of those who mainly eat red meat and mainly eat vegetables. Thus, for a balanced intestinal microbiota, a balanced and good diet is of great importance.
Impact of probiotics on microbiota
Probiotics are living microorganisms that have health benefits when consumed and often contribute to the diversity of intestinal microbiota. This modern definition is often attributed to a Nobel laureate, Élie Metchnikoff, who suggested a century ago that people who eat yogurt live longer. Consumption of probiotics is usually considered safe. Despite the negative impacts of the absence or impaired balance of the intestinal microbiota, the consumption of beneficial bacteria, namely probiotics, through nutrients may play a role in maintaining the normal balance in the intestine. It is also known that nutrients called prebiotics, which help beneficial bacteria to proliferate, may be beneficial.