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.
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