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Mechanism of synergy between plant essential oils and organic acids to promote animal health

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Abstract: With the advancement of the domestic ban, the research on antibiotic substitute products is becoming more and more active. Green additives such as plant essential oils, acidifiers, microecological preparations, probiotics and enzyme preparations have improved animal performance and maintained intestinal health. The role of the body's immune function has been widely used, but due to the complex and variable conditions of actual production, a single feed additive can no longer meet the production needs. Therefore, the application of synergistic effects between different additives has emerged. The mechanism of plant essential oils and organic acids to inhibit harmful bacteria, enhance the immunity of the body and the synergistic effect of the two are reviewed in order to provide reference for the application of poultry production.

Key words: plant essential oil; organic acid; animal health; mechanism of action


First, plant essential oil

Plant essential oil is an important active substance in aromatic plants, which is obtained by distillation, pressing and the like. The chemical composition of natural plant essential oils is very complex. It is found that there are more than 22,000 kinds of compounds constituting essential oils, which are mainly divided into four categories: terpene derivatives, aromatic compounds, aliphatic compounds and nitrogen-containing sulfur compounds. Although there are so many kinds of essential oils in the plant, the essential oils commonly used in production are mainly thymol, carvacrol, cinnamaldehyde, eugenol, linalool, eucalyptus, fennel, allicin and chili oil.

1.1 Maintaining intestinal flora balance

A large number of in vitro bacteriostatic test results show that active components in plant essential oils such as thymol, carvacrol, linalool and cinnamaldehyde can inhibit the growth and reproduction of microorganisms including bacteria, fungi, viruses and protozoa. Most are animal or foodborne pathogenic microorganisms. In order to compare the in vitro antibacterial effects of different plant essential oils and their components, Burt (2004) summarized the previous studies, listed the minimum inhibitory concentration of some plant essential oils on common foodborne pathogens, see Table 1, by table It can be seen that the bacteriostatic effect of thymol and carvacrol on Gram-positive and Gram-negative bacteria is comparable to that of rosemary, oregano and clove essential oil. Ouwehand (2010) reported that oregano essential oil, rosemary essential oil, thyme essential oil, carvacrol, cinnamaldehyde, citral, limonene and thymol can inhibit pathogenic Escherichia coli and Salmonella typhimurium in the intestine. The amount is inhibited by intestinal beneficial bacteria such as Lactobacillus reuteri and Bifidobacterium breve. Therefore, it is predicted that the essential oil component of the plant as an additive can affect the intestinal microbial composition of the animal.

The antibacterial mechanism of plant essential oils has been explored by many scholars. Table 2 lists the antibacterial mechanisms of several common essential oils and their components. It is generally considered that the bacteriostatic ability of the plant essential oil component is roughly as follows: phenols > aldehydes > ketones > alcohols > ethers > hydrocarbons. The lipophilic component of phenolic substances can aggregate in the lipid bilayer on the bacterial cell membrane, leading to disorder of bacterial cell membrane structure, enhanced permeability, outflow of cell contents, and ultimately bacterial cell death; aldehydes such as cinnamaldehyde It acts on the cell membrane and penetrates the cell membrane into the cytoplasm. It binds to proteins and inhibits key intracellular enzyme activities, such as aminodecarboxylase and adenosine triphosphatase, thereby preventing bacterial cell division. Terpenes mainly inhibit membrane activity. Causes local H+ concentration gradient and loss of electrochemical potential, thereby inhibiting bacteria.

1.2 Immunomodulation

The body's immune response is an important factor affecting the health of animals. Studies have shown that oregano oil can improve the body's immunity by promoting the growth of immune organs, increasing the immune organ index, and enhancing the synthesis of immunoglobulins. This may be due to oregano oil. The special aroma can stimulate the anterior lobes of the brain to secrete enkephalin and endorphin, and regulate the body's immunity against various pathogens and viruses. In addition, a variety of plant essential oils can regulate the body's immunity by regulating lymphocyte proliferation, phagocytic cells, cytokines, immunoglobulins and histamine synthesis and release.

In addition to regulating the immune response, the body can also improve the body's immunity by relieving the inflammatory response. Studies have shown that certain plant essential oils have anti-inflammatory effects, but different essential oils have different anti-inflammatory pathways, such as thymol can inhibit pro-inflammatory factors. For example, the synthesis of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), and the synthesis of anti-inflammatory factors such as interleukin-10 (IL-10); Lemon essential oil exerts an anti-inflammatory effect by inhibiting 5-lipoxygenase (5-LOX).

The anti-inflammatory mechanism of plant essential oils is mainly achieved by blocking the nuclear factor-kappaB (NF-κB) pathway. Under normal conditions, NF-κB and nuclear factor inhibitor κB (I-κB) bind to form an inactive complex. It is present in the cytosol, and after stimulation, I-κB is phosphorylated or degraded to release NF-κB and migrate into the nucleus. NF-κB undergoes a series of activities in the nucleus to trigger inflammation: on the one hand, it promotes inflammatory factor TNF- The expression of α, IL-1β, IL-6, on the other hand, can determine the inducible nitric oxide synthase (iNOS) and prostaglandin E2 (PGE2) that regulate the inflammatory process. In addition, plant essential oils also have a variety of anti-inflammatory pathways. For example, the essential oil of A. sinensis can protect IκB-α from phosphorylation and degradation. On the one hand, it directly blocks the release of NF-κB, and on the other hand, it indirectly leads to the nuclear endogenous subunit. Insufficient NF-κB synthesis is blocked, thereby inhibiting the expression of pro-inflammatory factors and the activity of related regulatory enzymes, and ultimately achieving anti-inflammatory effects; Shui Wen essential oil can inhibit the expression of TNF-α, IL-1β and its mRNA and NF- Nuclear migration of κB and p65; citral prevents IκB-α degradation, inhibits iNOS and NO production, and blocks nuclear migration of p50; α-hopsene and trans-syringene all reduce PGE2, iNOS and epoxidation The expression of enzyme-2 (COX-2), but α-hopnin can inhibit the production of TNF-α and IL-1β, while trans-syringene can only reduce the production of TNF-α.

1.3 Antioxidant

Oxidative stress means that under certain conditions, free radicals and related substances inside and outside the cell are produced in large quantities. The body's antioxidant system cannot remove these free radicals in time. These unremoved free radicals attack the polyunsaturated fatty acids on the cell membrane and cause cell membranes. Impaired structure and function; attacking proteins cause changes in enzyme activity, making the body's tissue function impossible, reducing immunity, inducing various diseases; attacking nucleic acids causes cytotoxicity and genotoxicity.

Under normal physiological conditions, the body's antioxidant system can maintain the generation and elimination of oxygen free radicals in a dynamic equilibrium. However, due to the emergence of modern intensive farming methods, breeding environment, disease, feeding management and other phenomena have caused continuous stimulation of animals, making animals oxidative stress for a long time.

Studies have shown that many plant essential oils have antioxidant effects, such as oregano, thyme, rosemary, olive leaves, marjoram, sage and other plant extracts have better antioxidant effects, their strength and essential oils The content and composition are related, among which the main antioxidant components are: thymol, carvacrol, cinnamaldehyde, cinnamyl acetate, citronellol, citronellal, β-myrcene and p-thymol. Its antioxidant mechanism is mainly through the following ways: First, directly remove 1,1-diphenyl-2-trinitrophenylhydrazine (DPPH), hydrogen peroxide (H2O2), hydroxyl ions (·OH), superoxide compounds, A series of free radicals such as superoxide anion, and the second is to prevent lipid peroxidation by inhibiting lipid oxidase activity, resisting linoleic acid oxidation, chelated metal ions and reducing iron ions. Usually a plant essential oil can work simultaneously through one or more of the above antioxidant pathways.

1.4 Interaction between plant essential oils

Due to the difference in the antibacterial mechanism of different plant essential oils, different essential oil components often have synergistic antibacterial effects. For example, although the antibacterial effect on cymene is weak, it can enhance the antibacterial effect of carvacrol by enhancing the swelling effect of carvacrol on bacterial cell membrane. In addition, thymol/carvacrol and cinnamaldehyde have synergistic antibacterial activity against Salmonella typhi and Escherichia coli. Zhou (2013) believes that this may be due to thymol/carvacrol which can increase plasma membrane permeability and promote cinnamaldehyde. Smooth passage of cell membranes or thymol/carvacrol increases the size and number of pores formed by the binding of cinnamaldehyde to proteins on the bacterial membrane.

Second, organic acids

At present, the commonly used organic acids are formic acid, acetic acid, propionic acid, butyric acid, lactic acid, benzoic acid, citric acid, fumaric acid, tartaric acid and sorbic acid.

2.1 Maintaining intestinal flora balance

Intestinal beneficial bacteria such as lactic acid bacteria and bifidobacteria can be propagated in an acidic environment, while harmful bacteria such as Escherichia coli (optimal pH: 6.0 to 8.0), staphylococcus (optimal pH: 6.8 to 7.5), and Salmonella (optimal) pH: 6.0 to 7.5) is inhibited from growth and reproduction in an acidic environment. Therefore, an acidic intestinal environment is conducive to the formation of a beneficial microflora.

Large dissociation of organic acids such as citric acid and lactic acid can release H+ in the intestine, lower the pH of the digestive tract, and form an acidic environment that is not conducive to the growth and reproduction of harmful bacteria, thereby achieving an indirect bacteriostatic effect. The undissociated organic acid with low dissociation can directly pass through the bacterial cell membrane. Once inside the bacterial cell, in the alkaline cytoplasmic environment, the organic acid dissociates to release H+ and RCOO-, on the one hand, H+ decreases. The intracellular pH of the bacteria, the bacteria maintain the intracellular acid-base balance, and the hydrogen ions are excluded from the extracellular passage by the ATP pump. This process consumes a lot of bacteria energy, which causes the bacteria to die. On the other hand, RCOO- can inhibit the DNA in the bacterial cell nucleus. The process of synthesis with proteins causes the bacteria to lose their ability to proliferate. Because the dissociation ability of organic acid is weaker than that of inorganic acid, the direct sterilization ability is strong, but different organic acids have different dissociation degree and molecular weight, and the bactericidal ability is also different. For example, citric acid and lactic acid have higher dissociation degree and bactericidal effect. It is not as good as formic acid and acetic acid, and the small molecule acid can provide more molecules than the macromolecular acid, and the bacteriostatic effect is stronger.

2.2 Relieve stress

Studies have shown that citric acid and fumaric acid have anti-stress effects, mainly because both citric acid and fumaric acid belong to the intermediate of the Krebs cycle, which can directly enter the Krebs cycle to release energy, which is shorter than glucose and is in the animal. In the stress state, it can be used for emergency synthesis of ATP for the body to cope with external bad stimuli and improve the body immunity. In addition, citric acid can be oxidized and decomposed into CO2 and HCO3- in poultry, relieve respiratory alkalosis, and reduce the harm of high temperature heat stress to chicken; fumarate itself has a sedative effect, inhibiting nerve center and reducing animal activity. Respond to heat stress.

2.3 Synergism between organic acids

Due to the different cell wall or cell membrane structure of different bacteria, there are some differences in the bacteria that are mainly targeted by different acids. Table 3 shows the ability of direct inhibition of different organic acids. The inhibition of harmful bacteria by any acid and the activation of beneficial bacteria need to be carried out within a certain pH range. The combination of several organic acids that can function in different pH ranges broadens the bacteriostatic and The bacteriostatic zone makes the addition amount less, and the effect is more remarkable. It overcomes the shortcomings of single acidifier, large amount of addition, strong irritancy, and the synergistic effect of each component.

Third, the synergy between plant essential oils and organic acids

Qi Shengli et al. (2010) believe that the addition of butyric acid to plant essential oils helps to enhance the antibacterial effect of active ingredients in plant essential oils. ZHOU et al. (2007) showed that the combination of thymol, carvacrol and acetic acid and citric acid can significantly reduce the number of Salmonella typhimurium. In addition, the results of Zhou et al. (2013) showed that the bacteriostatic activity of thymol (100 mg / L) and acetic acid (0.10%) composition and carvacrol (100 ul / L) and acetic acid (0.01%) composition reached separate The bacteriostatic effect of 400mg/L thymol and 400ul/L carvacrol has proved that the essential oil and acidifier have strong synergistic antibacterial effect. The mechanism of action may be that after the essential oil of the plant is released in the intestine, the membrane permeability of the harmful bacteria is changed, and the structure is destroyed, so that other bacteriostatic substances in the environment, such as organic acids, are more likely to enter the interior of the bacteria to break the acid-base balance. At the same time, plant essential oil destroys the bacterial proton pump, further destroying the ability of bacteria to maintain acid-base balance, and better exerting antibacterial and bactericidal effects. In addition, compared with the simple antibacterial action of organic acids, plant essential oils have the functions of regulating immunity, anti-inflammatory and anti-oxidation in addition to bacteriostatic in the intestinal tract, and the regulation of animal functions is not possessed by other additive products. It can form a complementary mechanism with the action of organic acids, and improve the health of the body from the perspective of bacteriostatic and animal function regulation.


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