Regulatory effects of piglet intestinal microbial metabolites and probiotics

The intestine is the largest digestive and immune organ of animals, and it is also the site with the largest number and variety of microorganisms. Intestinal microorganisms have a close interaction with the host. On the one hand, the host provides a stable growth environment for intestinal microorganisms; on the other hand, intestinal microorganisms can help the host digest food, resist pathogenic bacteria, and produce short-chain fatty acids (short-chain fatty acids). Metabolites such as chain fatty acids (SCFAs), bile acids, and vitamins, thereby regulating the host's physiological health and affecting disease development. This article mainly reviews the composition and distribution of intestinal microorganisms in piglets, the types and functions of intestinal microbial metabolites, and the impact of probiotics on the composition and function of intestinal microorganisms in piglets, with a view to providing insights into the research on intestinal microorganisms and diseases in piglets. Provide scientific basis for prevention and treatment.

 

1Piglet intestinal microorganisms

The gastrointestinal tract of piglets is sterile before birth. After piglets are born, some specific facultative anaerobic bacteria (such as Escherichia coli and Streptococcus) from the maternal birth canal and environment quickly colonize the intestines and create an anoxic environment, and then anaerobic bacteria (such as Bacteroidetes, Lactobacilli, Bifidobacteria, and Clostridia) begin to colonize, forming a simple microflora. The intestinal microorganisms of piglets are affected by the genetic background of the host, diet composition and growth environment. The intestinal microorganisms of piglets before weaning are mainly anaerobic bacteria. After weaning, the intestinal microorganisms of piglets continue to change with the growth and development of piglets, and eventually establish a harmonious relationship with the host. Symbiotic relationship. Studies have shown that the microorganisms in the intestines of piglets are mainly anaerobic bacteria and facultative anaerobic bacteria, most of which belong to the phylum Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria and Verrucomicrobia, among which Firmicutes and Bacteroidetes accounted for more than 90%. There are significant differences in the types and quantities of microorganisms in different intestinal segments of piglets, and there are also differences in the types and quantities of luminal microorganisms and mucosal microorganisms. Prevotella, Faecalibacterium, and Clostridium tenella are more abundant in the intestinal mucosa than in the intestinal content. things. The small intestine has the characteristics of higher pH, oxygen content and antibacterial substance levels than the large intestine and shorter retention time of chyme, which determines the rapid growth of facultative anaerobic bacteria (Clostridia and Proteobacteria) in the small intestine. Mainly, it has strong acid and bile salt tolerance, and the number of microorganisms in the small intestine is 104 to 107 CFU/mL. The large intestine is the site with the largest number of microorganisms in the intestine, dominated by Bacteroidetes and Clostridium, with the number of microorganisms reaching 1011 CFU/mL.

 

2 Metabolites of intestinal microorganisms

Intestinal microorganisms produce a series of metabolites through their own metabolism and co-metabolism with the host (Table 1), such as short-chain fatty acids, bile acids, polyamines, indoles and vitamins, etc., thus affecting the physiological health of the host. Currently, metabolomic methods such as gas chromatography (GC), liquid chromatography (LC), and high-performance liquid chromatography (HPLC) can usually be used to detect these metabolites.

 

 

 

 

2.1 Short-chain fatty acids

 

Short-chain fatty acids refer to organic fatty acids with less than 6 carbon atoms in the carbon chain. Short-chain fatty acids containing 2 to 6 carbon atoms are also called volatile fatty acids, including acetic acid, propionic acid, isobutyric acid, butyric acid, and isobutyric acid. Valeric acid and valeric acid, mainly acetic acid, propionic acid and butyric acid (95%), are mainly composed of fiber, a small amount of protein and peptides and other nutrients that cannot be digested and absorbed by the small intestine, and are fermented by anaerobic microorganisms in the colon and cecum. effect occurs. Short-chain fatty acids are the main energy source of colon epithelial cells and can provide 2% to 10% of energy for piglets. Most of the short-chain fatty acids produced in the intestine (80% to 90%) are absorbed in the colon, and only a small part of them are utilized by the microorganisms themselves, such as Desulfotomaculum spp. There are differences in the production of short-chain fatty acids in different parts of the intestine. The cecum and front colon have the highest short-chain fatty acid content (70-140 mmol/L), while the terminal colon has a significant decrease in short-chain fatty acid content (20-40 mmol/L). Studies have shown that short-chain fatty acids can directly activate G protein-coupled receptors (GPR) in intestinal epithelial cells, including GPR41, GPR43 and GPR109, thus regulating energy utilization, intestinal motility, cell proliferation and intestinal microecological balance of piglets. play an important role.

 

 

 

2.2 Bile acids

 

Bile acids are mainly synthesized from cholesterol in the liver and secreted into bile. As an important component of bile, bile acids play an important role in fat metabolism and can effectively increase the absorption of cholesterol and fat-soluble vitamins. According to the source, bile acids can be divided into primary bile acids and secondary bile acids. Primary bile acids are bile acids synthesized by liver cells directly from cholesterol as raw materials and catalyzed by cholesterol 7α-hydroxylase (CYP7A1) and sterol-27-hydroxylase (CYP27A1), including cholic acid. (cholic acid, CA), chenodeoxycholic acid (CDCA) and corresponding conjugated bile acids. Primary free bile acids are further degraded by dehydroxylation of bile salt-inducing enzymes under the action of intestinal bacteria into secondary bile acids, including deoxycholic acid (DCA) and lithocholic acid (LCA), which are respectively It is obtained by deoxygenating cholic acid and chenodeoxycholic acid through 7α-hydroxyl. Most of the bile acids (90% to 95%) are reabsorbed in the terminal ileum and reach the liver through the portal vein for enterohepatic circulation; the remaining bile acids (5% to 10%) are mainly absorbed by most anaerobic microorganisms (pseudophilic acid) in the intestine. Bile salt hydrolase (BSH) secreted by Bacillus, Clostridium, Eubacteria, Lactobacilli, Escherichia coli, etc.) and a few aerobic bacteria (Actinomycetes, Proteobacteria, etc.) hydrolyzes into primary free bile acids, only a few Some bile acids are excreted directly from the body in the feces. Most secondary bile acids (such as deoxycholic acid) are absorbed through transport carriers in the ileal epithelial cells or directly passively absorbed through the intestines. The reabsorption rate of lithocholic acid is low and most of them are excreted through feces. The synthesis and metabolism pathways of bile acids are shown in Figure 1. Studies have confirmed that primary and secondary bile acids regulate host glucose metabolism, lipid metabolism and energy metabolism by activating farnesoid X receptor (FXR) and G protein-coupled transmembrane receptor 5 (TGR5) respectively. Studies have found that clinical diseases such as inflammatory bowel disease, irritable bowel syndrome, and short bowel syndrome are all associated with abnormal bile acid levels.

 

 

 

 

2.3 Choline metabolites

 

Intestinal microorganisms can metabolize choline to produce trimethylamine oxide (TMO), which is then metabolized by liver flavinmonooxygenases (FMOs) to produce trimethylamine-N-oxide (TMAO). Studies have found that mice whose diets are supplemented with TMAO or are rich in choline, carnitine and γ-butyrylbetaine can accelerate the formation of thrombus, reduce the synthesis of primary bile acids by reducing the activity of CYP7A1, and increase the risk of cardiovascular diseases. risk. The study found that a high-choline diet significantly increased the abundance of Rhodobacterium, Veillonella, and Mycoplasma in the cecal lumen of mice, which was closely related to the high levels of TMAO in the mice. The study also found that TMAO regulates endogenous The release of calcium ions (Ca2+) from cells excessively increases platelet activity, which may increase the risk of atherosclerosis.

 

 

 

2.4 Other metabolites

 

Tryptophan is an essential amino acid for piglet growth. Intestinal microorganisms can metabolize tryptophan to produce kynurenic acid, serotonin, melatonin, indole, indoleic acid, skatole, tryptamine and other metabolites. These metabolites can be passed through Binds to the aryl hydrocarbon receptor (AhR) to promote host immune homeostasis. 1% to 2% of the tryptophan in the diet can be converted into serotonin and melatonin. Serotonin, as a neurotransmitter, can control gastrointestinal motility, sensitivity and secretion and can affect appetite and mood. Escherichia coli (E. coli) can metabolize tryptophan to produce indole, which can reduce the expression of the pro-inflammatory factor interleukin-8 (IL-8) and increase the expression of the anti-inflammatory factor interleukin-10 (IL-10) , strengthen the intestinal epithelial barrier function and reduce the colonization of pathogenic bacteria. In addition, there is a specific bacterium in the intestine, Clostridium sporogenes, which can specifically metabolize aromatic amino acids to produce indolepropionic acid. Indolepropionic acid has been shown to reduce intestinal permeability and regulate immunity. Cell activity and other effects.

 

3 Effects of probiotics on intestinal microorganisms of piglets

Probiotics refer to the general term for active microorganisms that are fed directly to animals and prevent diseases, promote growth, and improve feed utilization by regulating the balance of the animal's intestinal microecology. They mainly include three categories: Bacillus, Lactobacillus, and Saccharomyces. Studies have found that probiotics may compete with pathogenic bacteria for intestinal adhesion sites and nutritional substrates, and reduce intestinal pH through metabolism to produce lactic acid, acetic acid, etc. to prevent pathogenic bacteria from colonizing the intestinal tract, and can also regulate the intestinal microbial community of piglets. structure, increase the metabolism of nutrients, control pathogenic bacteria, etc., thereby promoting intestinal health of piglets.

 

 

 

3.1 Regulate the structure and diversity of intestinal flora

 

Studies have shown that adding Saccharomyces cerevisiae significantly increased the abundance of microbial flora in the cecum and colon of piglets before and after weaning, and was positively correlated with a variety of bacterial genera in the cecum and colon. Previous research by our research group also found that active dry yeast can significantly reduce the pH of the ileum and colon contents of early weaned piglets, and significantly reduce the number of E. coli. The complex of Enterococcus faecium and lactic acid bacteria significantly increased the number of lactic acid bacteria in the feces of weaned piglets, promoted material metabolism and energy metabolism, and the effect of Enterococcus faecium was better; in addition, Enterococcus faecium can also reduce the amount of Escherichia coli and demand in the small intestine of piglets. The number of aerobic bacteria. Bacillus amyloliquefaciens can increase the number of lactobacilli and bifidobacteria in the ileum of newborn piglets and reduce the number of Escherichia coli in the jejunum. Studies have found that adding Bifidobacterium infantis and Bifidobacterium lactis to the diet can effectively reduce the number of Salmonella in the feces of weaned piglets treated with Salmonella typhimurium and alleviate Salmonella infection. Probiotic Escherichia coli can secrete microcin, inhibit the growth of Salmonella and Escherichia coli in the inflamed intestines, and relieve Salmonella infections. Long-term supplementation of Enterococcus faecium to pigs can prevent intestinal infection by enterotoxigenic Escherichia coli (ETEC). Trevisi et al. found that supplementing Lactobacillus rhamnosus to weaned piglets treated with ETEC K88 did not affect the number of lactic acid bacteria and yeasts in the intestinal tract of piglets, but increased the number of ETEC in the feces, damaging intestinal health.

 

 

 

3.2 Regulate the production of intestinal microbial metabolites

 

Studies believe that part of the probiotic effects of probiotics are related to the regulation of metabolites produced by intestinal flora, and there are many research reports on short-chain fatty acids and bile acids. For example, Clostridium butyricum, as a butyric acid-producing bacterium, can increase the number of Megacoccus and Eubacterium halleri in the feces of weaned piglets, enhance the production and utilization of acetic acid, and produce more butyric acid, thus regulating the intestinal health of weaned piglets. . Adding compound probiotic preparations to sow diets during pregnancy and lactation can significantly increase the content of short-chain fatty acids in piglet feces. At the same time, it was found that the abundance of short-chain fatty acid-producing bacteria (Clostridium groups IV and XIVa) also increased significantly. high. This research group also previously found that there are significant differences in the intestinal microbial metabolites (especially short-chain fatty acids and bile acids) of different breeds of piglets. Among them, short-chain fatty acids and secondary bile in the colon contents of Landrace piglets that grow faster The acid content is significantly higher than that of plum blossom pigs that grow slower. As signaling molecules, short-chain fatty acids can regulate the host's fatty acid metabolism, sugar metabolism, and cholesterol metabolism by binding to their GPR receptors. They can also activate intestinal L cells to secrete glucagon-like peptide (GLP-1), thereby regulating insulin secretion. Released to affect feeding and growth. The latest research shows that early antibiotic treatment will destroy the intestinal flora balance and metabolic homeostasis of the ileum and cecum of suckling piglets, especially reducing the production of short-chain fatty acids and increasing the production of protein fermentation products (such as putrescine, cadaverine, etc.). However, feeding weaned piglets with the compound probiotic EBS (Enterococcus faecium, Bacillus and yeast) significantly increased the acetic acid and propionic acid contents in the piglet feces, while the compound probiotic EBL (Enterococcus faecium, Bacillus and paracasei Lactobacilli) significantly increased the contents of acetic acid, propionic acid, butyric acid and valeric acid in piglet feces, thereby improving the production performance of weaned piglets and reducing diarrhea rates.

 

In addition, studies have also shown that oral administration of probiotics with strong bile acid-binding ability (such as Lactococcus lactis) can promote the dissociation of bile acids in the small intestine, thereby promoting the reabsorption efficiency of the dissociated bile acids in the large intestine. Lactobacillus plantarum can significantly up-regulate the expression level of CYP7A1 gene in mice, while increasing the excretion of bile acids in mouse feces, thereby reducing the concentrations of low-density lipoprotein and triglycerides in mouse serum. Probiotics may regulate bile acid metabolism by regulating the activities of bile salt hydrolase (BSH) and bile acid-inducing enzyme (BAI), causing the dissociation and dehydroxylation reaction of bile acids, thereby affecting the host's physiological cholesterol metabolism process. In addition, bile acids have also been shown to improve intestinal integrity and growth performance in early weaned piglets.

 

4 Summary

The intestine is a place where microorganisms, nutrients and immune cells are fully in contact. Intestinal microorganisms have probiotic effects such as digesting and fermenting carbohydrates, maintaining normal intestinal functions, regulating immunity, and competitively inhibiting pathogenic bacteria, thus regulating the growth of piglets. These effects may be Intestinal microbes produce specific small molecule metabolites as downstream signaling molecules that bind to host receptors. Changes in intestinal microorganisms and metabolites can be studied through microbiomics, metagenomics and metabolomics technologies, and the correlation between intestinal microorganisms and host phenotypes can be analyzed through bioinformatics methods, but intestinal microorganisms and their metabolites How to affect host metabolism, especially the identification of key functional bacteria and the screening of key metabolites that affect host phenotypes are still difficult. The current research on the relationship between piglet intestinal metabolites and host health is still in its infancy. Therefore, sterile animal models and multi-omics combined methods are used to comprehensively analyze the changes in intestinal microorganisms and their metabolites, as well as the effects on host phenotypes. Causal relationship research is an important direction for future intestinal microbial research.