Intestinal flora refers to the flora that is designated to be planted in the human intestinal tract and is interdependent with the human body for a long time, including more than 40 genera, 400 to 500 species of bacteria, the number of which is up to 100 trillion, 10 times the number of human body cells, the weight of which is equivalent to a person's liver, accounting for about 1/3 to 2/5 of the feces emitted by healthy adults every day.

According to the distribution and existence of intestinal flora in the intestinal tract, it can be divided into two categories: membrane flora and cavity flora. Membrane flora, also known as native bacteria, can be colonized near the mucosal surface a few days after birth. It is a long-term resident bacteria, which still maintains a large number in adulthood and is closely related to the epithelium of the colonization site, such as bifidobacteria, anaerobic lactobacilli, bacteroides, etc; Cavity flora, also known as foreign bacteria, mainly exists in the intestinal cavity and is a small number of unstable passing bacteria, such as Pseudomonas aeruginosa, enterococcus, Escherichia coli, etc. Once the number of cavity flora increases, it will cause disease.

According to the relationship between intestinal flora and human body, we artificially divide them into three categories: the first is our friends, namely, beneficial bacteria, such as bifidobacteria, lactobacillus acidophilus, etc; The second is our enemies, namely harmful bacteria such as Staphylococcus, Pseudomonas aeruginosa, etc; The third type is intermediate molecules, i.e. intermediate bacteria, such as enterococcus, Escherichia coli, etc. There are individual differences in the composition and quantity of intestinal flora among different people, which may be controlled by individual heredity.

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