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Probiotics Regulate Gut Microbiota
Probiotics are beneficial active microorganisms that colonize the human intestines and change the composition of the flora in particular parts of the host. Recent evidence has shown that probiotics play significant roles in gut microbiota composition, which can inhibit the colonization of pathogenic bacteria in the intestine, help the host build a healthy intestinal mucosa protective layer, and enhance the host immune system. Based on the close relationship between the gut microbiota and human immunity, it has become an extremely effective way to improve human immunity by regulating the gut microbiome with probiotics.
2. The Mechanisms of Probiotics to Exert Their Beneficial Effects
2.1. Regulation of Intestinal Flora by Probiotics
2.2. Maintenance of the Epithelial Barrier
2.3. Inhibition of Pathogen by Probiotics
There are a large number of microorganisms in the human intestine, including pathogenic microorganisms and probiotics. The homeostasis of the gut microbiota may be altered by pathogenic microorganisms, which can elevate the risk of developing related diseases . In past studies, it has been well documented that probiotics can protect the intestinal tract by inhibiting pathogenic bacteria. Therefore, the effects of probiotics on inhibiting pathogens in the gut and their underlying mechanisms have received significant attention from the research community. There are many mechanisms for probiotics to inhibit pathogenic microorganisms, such as stimulation of epithelial barrier function, producing antimicrobial substances, limiting access of pathogenic microorganisms to nutrient resources, and competitive exclusion by competition for binding sites . Conversely, a crucial beneficial mechanism of probiotics is the competitive exclusion of pathogens . For example, the experiment by Fang et al.  showed that probiotic Escherichia coli Nissle 1917 (EcN) can secrete DegP (a bifunctional periplasmic protein) to inhibit enterohemorrhagic E. coli (EHEC). The probiotic Escherichia coli outcompetes pathogenic biofilms via extracellular DegP activity during dual-species biofilm formation. In another experiment, probiotics can secrete antibacterial substances, causing steric hindrance and competitive adhesion sites and nutrients to prevent Helicobacter pylori from binding to epithelial cells . On the other hand, secreting antibacterial compounds is another vital function of probiotics. A typical example is that probiotics can secrete organic acids during carbohydrate fermentation, such as butyric acid, acetic acid, and propionic acid. Organic acids have been considered to be the main antimicrobial compounds responsible for their inhibitory activity against pathogens. The decrease in pH and the presence of undissociated acid make organic acids have a certain antibacterial activity . Interestingly, some experiments also found that through inhibition of the pathogen signal system, intestinal pathogens can be eliminated.
2.4. The Modulation and Proper Maturation of the Immune System
3. Effect of Gut Microbiome on Immunity
3.1. The Role of Gut Microbiome in the Immune System
As is known, the gut has an immune system that can prevent the invasion of pathogenic microorganisms and create a suitable environment for beneficial bacteria, which plays a crucial role in maintaining host health. Many studies have proven that the intestinal microbiome can strengthen the intestinal immune system , indicating intestinal flora plays an important role in the immune system.
3.2. Gut Microbiome, Food Allergy, Cancer and Depression
The study confirmed that in multiple interconnected networks of host immunity and homeostasis, the gut microbiota plays an important role in maintaining homeostasis . The gut microbiota is involved in structural modification of the host intestinal mucosa, neurotransmission, vitamin K production, as well as the development of immune responses . Currently, many experiments have suggested a role for the gut microbiota in the pathogenesis and progression of food allergy , and gut microbes can function to maintain the immune system’s efficiency .
The origin of cancer is a malignant tumor of epithelial tissue; the generation of cancer is generally thought to be secondary to a state of local chronic inflammation . The occurrence of cancer is directly related to immune deficiency. With the discovery of the beneficial effect of intestinal flora on immunity, the relationship between the intestinal microbiome and cancer has been paid more and more attention . The most typical example is the relationship between intestinal flora and colorectal cancer. Colorectal cancer has a high incidence and mortality . Through research in recent years, it has been proven that there is a close relationship between intestinal microorganisms and colorectal cancer. The study found that specific species of bacteria may affect both the risk of colorectal cancer and the growth of existing tumors . For instance, the experiment by Ahn et al.  showed that microbial diversity was significantly lower in the gut of patients with colorectal cancer. More specifically, colorectal cancer patients had higher numbers of Fusobacterium and Porphyromonas and a lower relative abundance of clostridia. This result has also been confirmed in other experiments; other studies have found that survival of patients with colorectal cancer is associated with the concentration of Fusobacterium nucleatum .
The common beneficial microorganisms in the gut include Bifidobacterium sp., Streptococcus thermophilus, Lactobacillus sp., and Saccharomyces boulardii . They have become a regular part of our dietary habits . The gut microbiome may influence normal brain development, mood, and pain sensitivity. Moreover, several research findings have shown that probiotics can affect the CNS by regulating the intestinal flora and preventing mental diseases through the gut–brain axis . Recently, it has been proven that these beneficial microorganisms in the intestine can enhance immunity, and the improvement of immunity helps prevent the occurrence of depression. Depression is the most common disorder. The main clinical manifestations are depression, slow thinking, and impaired cognitive function. With the continuous development of science and technology, metagenomics and molecular tools have been further improved, contributing to the continued promotion of research . In studies of depressed patients, it was found that cell-mediated activation of adaptive immunity appears to be dramatically different in depressed patients compared to the general population . Moreover, in past studies, there have been a large number of experimental results that have proven that the activation of innate immune mechanisms, such as the activation of the proinflammatory cytokines interleukin-6 and interleukin-1, has a certain association with the development of depression . The link between the occurrence of depression and the immune system is obvious. The intestinal flora has the function of enhancing immunity and refining the immune system, which has a certain theoretical basis for advocating that intestinal flora can improve immunity to treat depression.
4. Probiotics, Gut Microbiome, Immunity and Life
4.1. The Role of Gut Microbiome in Obesity and Local Inflammation of Adipose Tissue
4.2. The Relationship between Immunity and Sleep and the Effect of Gut Microbiome on Sleep
4.3. The Relationship between Intestinal Flora and Skin and the Use of Probiotics to Improve Skin Quality
This entry is adapted from 10.3390/molecules26196076
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