Plant extracts are substances derived from plants that have one or more biological functions. They have been shown to have various effects when added to feed, such as: effectively improving the growth rate of livestock and poultry, improve the quality of livestock meat, enhancing immunity, and ensure intestinal health, etc. Plant extracts contain rich and complex organic components, which have been proved to have bioactive substances including flavonoids, plant essential oils, alkaloids, tea polyphenols, saponins and polysaccharides. Flavonoid is a kind of compound with 2-phenylchrominone structure existing in plants. Their molecules contain one ketone carbonyl group. The oxygen atom on the first position is alkaline and can form salt with strong acid. Alkaloids are an important nitrogenous organic compound in natural plants, these alkaline and can be combined with acids to form salts. It has been proved that alkaloids in natural plants have antibacterial biological activities. Essential oils (EOs) are widely found in natural plants. They have the advantages of antibacterial, antioxidant and lipid-lowering, and have been widely added to feed additives.

How does the antibacterial activity of plant extracts work?

  • Enhance intestinal epithelial defense function

It has been confirmed that some plant extracts do not directly inhibit pathogenic bacteria in vitro, but can resist diarrhea caused by pathogenic microorganisms after ingestion of livestock and poultry, which may be mainly achieved by improving the self-defense function of intestinal epithelium. For example, allicin cannot directly inhibit E. coli, but can enhance the resistance of enterocyte to E. coli K88 infection and reduce the permeability of intestinal epithelial cells caused by E. coli.

  • Enhance bacterial cell membrane permeability

An important characteristic of plant extract is its good hydrophobicity, which allows it to separate the phospholipid structure of the bacterial cell membrane from that of the mitochondria, damaging the cell structure and enhancing the permeability of the cell membrane, thus leading to the leakage of ions and other substances inside the cell. If a large number of cell contents or important ions or molecules leak out of the cell, it may lead to bacterial cell death. Lambert et al. have confirmed that carvol from oregano can cause the release of phosphoric acid molecules in the cells of Staphylococcus aureus and Pseudomonas aeruginosa, and that the inhibitory or killing properties of carvol are related to its lipophilic properties and aromatic chemical structure.

  •  Inhibit the synthesis of bacterial peptidoglycan

Some in vitro studies have shown that plant extracts can inhibit the synthesis of peptidoglycan and damage the morphology of bacteria. However, the components of plant extracts are complex and the specific antibacterial mechanism of plant extracts is still unclear. The main antibacterial mechanism of the extract is that it inhibits the synthesis of peptidoglycan and thus damages bacterial morphology.

In addition, plant extracts can also destroy or degrade bacterial cell wall, destroy cytoplasmic membrane and cell membrane protein structure, can also make bacterial cytoplasmic condensation, make bacterial proton movement weakened. The several antimicrobial mechanisms of plant extracts described above are not isolated, they interact with each other, and the reaction of one mechanism may be influenced by the reactants or products in the other mechanism. In addition, the non-major active ingredients in the plant extract also acted with the major active ingredients to enhance its effect.

Effects of plant extracts on intestinal health of poultry

Effects on intestinal digestion

Studies have shown that plant extracts can stimulate the secretion of digestive enzymes in poultry intestines, enhance the activity of digestive enzymes, and thus improve the digestive function of poultry intestines. Capsaicin and piperine promote digestion mainly by stimulating the activity of digestive enzymes. The main reason is that the active substances in these spice extracts can stimulate the secretion of bile salts.

William et al. showed that plant essential oil can increase the secretion of intestinal amylase and other endogenous enzymes of broilers, improve the activity of digestive enzymes in the intestine, change the shape of feed in the intestine and reduce the viscosity of intestinal chyme, so as to improve the digestibility of nutrients in the feed. Hawthorn extract can improve the activities of duodenal protease, pancreatic protease and lipase, and improve the digestibility of nutrients of broilers. Supplementation of 200mg/kg Yucca extract (YSE) in the diet of broilers significantly increased the activities of trypsin and lipase in ileum and duodenal chyme of broilers at 42 days of age, and significantly increased the apparent metabolic rate of crude protein of broilers. In addition, the seed extract of black cumin significantly increased pH of chicken muscle stomach, but had no significant effect on pH of glandular stomach, duodenum, ileum and jejunum. In conclusion, plant extracts may destroy the adhesion of pathogenic bacteria in the intestine by stimulating the gastrointestinal tract to secrete mucus (including some digestive enzymes), thus reducing the damage caused by pathogenic bacteria to the intestine and improving the digestion and absorption of intestinal nutrients.

Effects on intestinal development and mucosal morphology

Intestinal mucosa is the main site of nutrient absorption and the first immune barrier of the body. The integrity of the morphology, structure and function of the intestinal mucosa is of great significance to the maintenance of intestinal health and production of poultry. The results showed that 0.2% galactomano-oligosaccharide could increase the villus height and villus surface area of small intestine and reduce the risk of Salmonella typhimurium infection in broilers fed with pine extract containing 0.1%, 0.2% and 0.3% galactomano-oligosaccharide. Plant extracts can affect the intestinal adhesion of pathogenic bacteria, maintain the intestinal microecological balance of poultry, improve the mucosal morphology of jejunum and colon, and increase the villus height and crypt depth.

Alkaloids can promote the proliferation of some beneficial bacteria, inhibit the growth of harmful bacteria, promote the development of villus structure and improve the absorption function of small intestine. Dietary yucca extract can improve the villus height and villus height/crypt depth of ileum and cecum of broilers, improve the digestive and absorption function of intestinal mucosa, and promote the growth and development of broilers. Dietary plant extracts can increase the villus height of small intestine and crypt depth of ileum and jejunum of broilers. In addition, plant extracts also have the function of repairing damaged intestinal mucosa and regulating the expression of mucosal cytokines. After intestinal mucosal injury, intestinal repair is regulated by a variety of active growth factors, including epidermal growth factor (EGF), transforming growth factor α (TGF-α), transforming growth factor β (TGF-β), cell proliferation nuclear antigen (PCNA), etc.

Plant extracts may affect the morphology of intestinal mucosa by affecting the proliferation and differentiation of intestinal epithelial cells and the expression of intestinal epithelial-related genes. Studies on piglets demonstrated that plant essential oils down-regulated the expression of cyclin D1 mRNA, which is associated with cell proliferation, in jejunum and ileum, and reduced the villus height. Considering the different reports on the effects of plant essential oil on intestinal morphology, it can be hypothesized that one possible effect of plant essential oil is to stimulate intestinal tissue and damage intestinal surface, while other beneficial effects on intestinal health, such as reducing intestinal pathogenic microorganisms, may be conducive to increasing the height of intestinal villus and thereby improving intestinal morphology.

Effects on intestinal flora

Intestinal flora is an important part of the intestinal mucosal barrier, also known as the biological barrier. The imbalance of intestinal flora can cause diarrhea, low immunity and even systemic inflammatory response syndrome (SIRS) in poultry. The supplementation of 60, 120 and 180mg/kg yucca extract in the diets of broilers showed that 60 mg/kg yucca extract increased the number of Firmicutes in intestinal tract, and 120 and 180mg/kg yucca extract decreased the number of firmicutes in intestinal tract. 60 mg/kg Yucca extracts reduced the number of Bacillus in intestinal tract, and 120 and 180 mg/kg yucca extracts increased the number of Bacillus in intestinal tract. 60 and 180 mg/kg Yucca extracts reduced the number of Proteobacteria in intestinal tract, and 120 mg/kg yucca extracts increased the number of Proteobacteria in intestinal tract. The number of harmful bacteria such as Bacteroides and Eubacter decreased with the increase of the yucca plant extract, while the number of Helicobacter increased with the increase of the yucca plant extract. The number of beneficial bacteria such as Ruminococcus, Clostridium teniculae and Miraria in intestinal tract increased with the increase of yucca extract supplemental level.

Kavoosi et al. studied the antibacterial effects of essential oil of fennel seed from India (fennel essential oil) and essential oil of Ferro ferro from bamboo (Ferro ferro essential oil), and found that fennel essential oil could significantly inhibit the growth of gram-positive and gram-negative bacteria, while Ferro essential oil could only significantly inhibit the growth of gram-positive bacteria. In addition, they also found that both of these two plant essential oils had antifungal effects. The addition of 2g/kg oregano water extract could significantly reduce the number of Escherichia coli in ileum and cecum, and the number of enterococcus, lactic acid bacteria and staphylococcus in cecum was not significantly different among groups, while the number of lactic acid bacteria in ileum did not decrease significantly. Supplementation of thyme essential oil in the diet of Japanese quail shows that the essential oil can significantly increase the number of Lactobacillus ileal and decrease the number of Escherichia coli. However, dietary supplementation of plant essential oil had no significant effect on small intestinal microflora of chickens aged 7 to 28 days.

The above studies showed that different plant extracts regulate intestinal flora in different ways, some can directly inhibit harmful bacteria, some can promote the growth of probiotics and indirectly inhibit the propagation of harmful bacteria. Plant oligosaccharides have obvious probiotic effects, which can promote the growth and propagation of beneficial bacteria such as bifidobacteria and lactic acid bacteria in vivo and in vitro, because anaerobic bacteria such as bifidobacteria and lactic acid bacteria can use oligosaccharides for anabolic metabolism, while aerobic pathogenic bacteria can not use oligosaccharides. Plant extracts from different sources have different mechanisms of action on intestinal microecology, and a variety of plant extracts may exhibit stronger or broader spectrum antibacterial effects.

Effects on intestinal immunity

As an important organ for the digestion and absorption of nutrients and a congenital immune barrier to maintain the stability of the internal environment, intestinal tract is the most complex immune system in the body. When the intestinal immune barrier is damaged, microorganisms and toxins can enter the blood circulation, thus causing the relocation of bacteria and toxins, easy to cause enterogenic infection, and even lead to systemic inflammatory reaction or organ function failure. It has been proved that plant extracts can bind to specific receptors on immune cell membranes, mediate the signaling pathway of immune cell activation, regulate the secretion of macrophages, T/B lymphocytes, dendritic cells and other cytokines, and maintain the intestinal health of animals by enhancing their immunity and eliminating inflammation (Figure 1). At present, it has been found that the cytokines that maintain intestinal health include interleukin-6 (IL-6), interferon-γ (IFN-γ) and TNF-α, which play a very important role in the process of inflammation and immune response, and also play an important role in the fight against various pathogens in the body.

Studies have shown that the extract of wild rose can significantly increase the expression of IFN-γ in the body, significantly reduce the expression of interleukin-4 (IL4), and increase the ratio of IFN-γ to IL-4, that is, wild rose extract can significantly improve the balance of T helper cells (Th) 1/Th2, and can be used as an immunomodulator in inflammatory reactions such as allergy. Qiweibaizhu powder can increase the mRNA expression levels of interleukin-2 (IL-2), interleukin-10 (IL-10) and IFN-γ in the intestinal mucosal epithelium of lactating mice, and inhibit the diarrhea caused by rotavirus. The mechanism may be related to the regulation of T cell subset. RomierCrouzet et al. studied the effects of grape seed extract, cocoa extract, sugarcane extract, oak extract, mangosteen extract and pomegranate extract on the inflammatory mediators of human intestinal cells. The results showed that sugarcane extract, oak extract and pomegranate extract could inhibit the activity of nuclear transcription factor κB (NF-κB). Pomegranate extract inhibited the activation of extracellular regulatory protein kinase (Erk) 1/2, oak extract and pomegranate extract decreased the synthesis of nitric oxide (NO) and interleukin-8 (IL-8), and pomegranate extract and cocoa extract decreased the synthesis of prostaglandin E2 (PGE2). In mice, plant extracts down-regulate the expression of NF-κB1 and NF-κB2 genes, which increases the expression of NF-κB1 inhibitors, increases the binding of NF-κB1, and releases NF-κB1 from the nucleus to the cytoplasm. The gene expression of nitric oxide synthase (NOS and cyclooxygenase (COX-2) is reduced, and the contents of inflammatory factors NO and PGE2 induced by NOS and COX-2 are also reduced correspondingly, thus reducing the inflammatory response of the body. The bidirectional regulation of plant extracts not only improves the immunity of infected animals but also prevents hypersensitivity resulting in autoinflammation. However, the exact mechanism of plant extracts on intestinal immunity or reducing intestinal inflammation in poultry remains unclear, and further studies are needed.

Effects on intestinal metabolites

Plant extracts can also reduce harmful metabolites in digestive tract and feces to maintain intestinal health and improve the environmental quality of farms. When the diets supplemented with 150, 250 and 350mg/kg lauraceae plant extracts, the contents of uric acid (UA) and urea nitrogen (UN) in intestinal contents of experimental groups were lower than those in control group, and the contents of UA in intestinal contents of experimental groups were decreased by 0.25%, 23.9% and 41.3%, respectively. The emission of ammonia (NH3) is reduced. The extracts of Lauraceae, garlic and Yucca were added to the diets of broilers. The results showed that at 42 days of age, the UN content in intestinal contents of broilers in the group was decreased by 31.08%, 28.34% and 28.52% compared with that in the control group, respectively. The UA content in intestinal contents decreased by 22.48%, 22.13% and 38.85% compared with the control group, respectively. The nitrogen (N) and dry matter (DM) digestibility of broilers were increased by adding 0.1% fenugreek seed extract in the diet. At the same time, 0.1% forsythia leaf extract (FSE) can also reduce the release of NH3 in feces. On the one hand, plant extracts reduce the harmful metabolites in intestinal tract by inhibiting the activity of xanthine oxidase (XO) and reducing the production of UA. On the other hand, plant extracts inhibited the activities of urease and uricase in the intestinal tract of poultry and reduced the contents of UA, UN and AN in the intestinal tract of poultry. The plant extracts excreted outside the digestive tract still inhibited urease and uricase and reduced the contents of UA, UN and AN in the excreta. In addition, some plant extracts can stimulate the secretion of protease in the digestive tract, improve the efficiency of protein utilization, thus reducing nitrogen emissions and improving environmental quality.

Write at the end

Plant extracts have both nutritional and health benefits and are an ideal substitute for antibiotics. At present, researchers have done a lot of work on the effects of plant extracts on digestive physiology, intestinal development, flora and harmful metabolites of poultry, but the mechanism of the effects of extracts from different sources, different extraction methods and different plant growth stages on the regulation of intestinal health of poultry remains unclear. The elucidation of its role in vivo and the molecular channels affecting the expression of intestinal mucosal-related genes at the cellular and molecular levels provided a theoretical basis for the application of plant extracts in poultry production.