Antimicrobial peptides are an important part of the mammalian defense system, with a small molecular weight, good thermal stability, no immunogenicity, and a broad antibacterial spectrum. It has the functions of promoting growth, health care, and treating diseases for livestock and poultry, and has no toxic side effects, no residues, and no bacterial drug resistance. This article reviews the classification of antibacterial peptides, antibacterial mechanism, application effects and application advantages as feed additives. Key words: antimicrobial peptides; feed additives; mechanism of action; application effects As antibiotics are widely used in feeds and people are increasingly demanding food and environmental quality, people are increasingly aware of the side effects of antibiotics, antibiotics in feed Applications in the industry have already faced elimination or banned situations. The use of non-toxic and non-pollution new antibacterial agents as feed additives instead of antibiotics has become an important part of the current domestic and foreign feed disciplines. With China's entry into WTO, antibiotics as feed additives will seriously affect the competitiveness of livestock products in the international market. The application of biotechnology to research and develop new antibacterial agents is an urgent task for the feed science community in China. Antimicrobial peptides have a broad-spectrum antibacterial effect, and have the functions of promoting growth, health care, and treating diseases in livestock and poultry. They are non-toxic, non-residual, and non-bacterial drug-resistant environmental protection agents. Bioengineering technology can be applied to produce transgenic animal and plant products that are resistant to pathogens. At the same time, a large number of antimicrobial peptides can be expressed through genetic engineering techniques, making it the source of a new generation of peptide antimicrobials, and has broad application prospects. 1 Classification of antimicrobial peptides 1.1 Mammalian antimicrobial peptides Mammalian antimicrobial peptides are widely found in neutrophils, mucous membranes, and skin. For example, PR-39 peptide rich in pro-arg, molecular weight 471 9ku, NK-lysin with 78 amino acid residues are present in pig intestine, and 5 small molecules are isolated from porcine leukocytes. Peptides, which contain only 16 to 18 amino acid residues; at least 10 antibacterial peptide genes have been found in sheep; at least 30 antibacterial peptides have been found in cattle; 3 antibacterial peptides have also been isolated from cow's milk, and they have suppression Enterotoxigenic Escherichia coli, growth of Listeria monocytogenes. Defensins are the most studied antimicrobial peptides in mammals and include two major classes: alpha-defensins and beta-defensins. It is mainly found in mammalian leukocytes, phagocytic cells or small intestine Paneth cells, especially in neutrophils. Defensins usually consist of 27 to 54 amino acid residues, and they form three stable intrachain disulfide bonds by conserving cysteine ​​to conjugate the peptides to form a stable antiparallel β-sheet structure. Mammalian defensins have 6 conserved cysteines and can form three stable intrachain disulfide bonds, which is one of the main features distinguishing other antimicrobial peptides. The antimicrobial spectrum of defensins is relatively extensive, including Gram-positive and negative bacteria, mycobacteria, spirochetes, fungi, and some enveloped viruses. In addition, it also has toxic effects on malignant cells. 1.2 Insect Antimicrobial Peptides Cecropins, an efficient antimicrobial peptide isolated from the immunohemolymph of the silkworm, is the earliest discovered antibacterial peptide. Since then, Cecropins or similar molecules have also been identified in many insects. Lee et al. also found a cecropin in the pig's intestine, which may suggest that cecropin is widely present in animals. Cecropins has strong activity against various E. coli and other Gram-negative bacteria, such as Salmonella, and also has killing activity against certain Gram-positive bacteria, but has no cytotoxic effect on molds and other eukaryotic bacteria. Cecropins is a family of linear cationic peptides, 35 to 40 amino acid residues in length, and the carboxyl amidation of the last residue at the C-terminus. The cDNA sequence analysis of Cecropins revealed that the precursor molecule contained 62 to 64 amino acid residues, and the carboxyl amidation of the last residue at the C-terminus. The cDNA sequence analysis of Cecropins revealed that its precursor molecule contains 62 to 64 amino acid residues, with a conserved signal sequence and leader sequence. There are 3 genes in silkworms and 4 in fruit flies. The expression of the gene is induced upon injury and infection. For example, within a few hours after the injection of bacterial debris or LPS, specific mRNA in the adipocytes increased significantly, and within 24 hours, the detection of cecropin in the hemolymph fluid was detected. After the natural immune cell membrane receptor recognizes the microbial component, it activates the NF-KB appearance through the Toll pathway. Bee Venom hemolytic peptide is a polypeptide in bee venom, which consists of 26 amino acid residues and has a molecular weight of 2480D. Bee venom hemolytic peptide can have a strong hemolysis effect directly with the membrane lipid of red blood cells. Apila is a short peptide extracted from the body fluids of honeybees. It is composed of 18 amino acids and has an efficient broad-spectrum antibacterial effect and is not toxic to eukaryotic cells. 1.3 The frog antibacterial peptide Zaslff was found in the Xenopus laevis skin as a small molecule antibacterial peptide called Magainins. Since then, a variety of frog antibacterial peptides have been discovered. For example, there are more than ten antibacterial peptides in Xenopus laevis, not only in the skin granules, but also in the gastric mucosa and small intestine cells. The frog antibacterial peptides have a synergistic effect, but different frog antibacterial peptides rarely have homology. 1.4 Thyins, a plant antimicrobial peptide, was the first antimicrobial peptide isolated from plants. They are toxic to Gram-positive bacteria, negative bacteria, fungi, yeast, and mammalian cells. Other plant antimicrobial peptides that are structurally similar to insects and mammalian defensins are called plant defensins. 1.5 Bacterial antimicrobial peptides Bacterial antimicrobial peptides include cationic peptides and neutral peptides, which are secreted by both Gram-positive and Gram-negative bacteria. For example, nisin is a short peptide containing 3 to 4 amino acid residues produced by Lactococcus. It is an acid-resistant substance and has high stability even in a low-pH environment such as stomach, which inhibits Gram Positive bacteria such as Clostridium and Listeria. 2 Antimicrobial peptide antibacterial mechanism The antibacterial peptide has a broad-spectrum bactericidal action. The antibacterial effect is mainly due to the formation of ion channels on the plasma membrane of prokaryotic cells under the action of the amphipathic alpha helix structure. The diameter of this channel is estimated to be approximately 4 nm. The formation of ion channels alters the osmotic pressure inside and outside the cell, causing massive extravasation of intracellular substances, and acts as a bactericidal effect. Jiang Ming et al. observed the effect of cecropin on the E. coli by electron microscopy and found that after a certain period of time, The trumpet-shaped gap formed on the outer wall of the bacteria indicates that the antibacterial peptide molecule is inserted into the bacterial cell membrane at one end and forms a channel by “drilling†to break the bacterial cell membrane and cause the protoplasm to “leak†and die. From this point of view, the interaction between the positive charge of the antimicrobial peptide and the negative charge of the phospholipid head of the bacterial membrane is crucial. The amphipathic nature of the α-helix at the N-terminus of the peptide molecule is the main part of the lytic bacteria, and the amidation of the C-terminus Broad-spectrum antibacterial related. It can be seen that the antibacterial mechanism of antibacterial peptides is completely different from the antibacterial mechanism of antibiotics that act by blocking the biosynthesis of macromolecules, and pathogenic bacteria are not easily resistant to them, so the substance will become the source of new antibacterial drugs. . 3 Application of antimicrobial peptides 3.1 Application in animal husbandry Piglet diarrhea, cow mastitis, and various viral diseases such as swine fever and Newcastle disease have always been thorny diseases that are not conducive to the development of animal husbandry. Reference has been made to successful insect antibacterial peptide transgenic projects, such as genetically modified mosquitoes, transgenic potatoes, transgenic rice, etc., to transfer specific antibacterial peptide genes into specific livestock and poultry cells for expression, thereby generating new disease-resistant varieties. According to reports from American scholars, antimicrobial peptides can be used as feed mold inhibitors. It was also reported that tussah silkworm antibacterial peptides are effective in preventing and treating chickens. The use of antibacterial peptide-containing tussah immune hematological lymphocyte powder was added to weaned piglets. Feeding test results showed that silkworm antibacterial peptides can reduce diarrhea in weaned piglets. Wen Liufa et al. (2001) reported that through a 50 L fermentor culture intermediate experiment, after optimizing the fermentation conditions, the alcohol oxidase promoter was induced with methanol to successfully express the antimicrobial peptide gene and secrete the antimicrobial peptide into the bacteria. The bactericidal activity of the antimicrobial peptides reached 5,000 units/ml. To further improve the medium formulation, to master the appropriate culture conditions, including PH, temperature, air supply, and agitation speed, the fermentation production cycle can be shortened to 24 hours and the bactericidal activity can reach 5600 units/ml. The experimental yeast fermentation broth was used as a test. The results of the feeding test of Guangdong yellow chicken with a drinking water intake of 60 u/d?d showed that the antibacterial peptide can promote the growth of chickens and reduce the nitrogen content of fecal material, and has a promoting effect on yellow chickens. , health care and treatment of diseases. The application effect of the silkworm antibacterial peptide yeast preparation as feed additive is significant. 3.2 Pharmaceuticals With the widespread and long-term application of traditional antibiotics, many pathogenic bacteria have developed resistance to them, and antimicrobial peptides with broad-spectrum antibacterial and unique antibacterial mechanisms clearly have obvious advantages in this area of ​​application. . With the deepening of the understanding of the relationship between the structure and activity of antibacterial peptides, the mechanism of antibacterial peptides and the mechanism of gene expression regulation, a highly effective antibacterial peptide that is beneficial to human health has become a reality. Silkworm antibacterial peptides have been developed and utilized in medicine, such as hepatitis B drugs that have been used for the treatment of nephritis, kidney cancer, and hepatitis B drugs approved for western medicine, and use antibacterial peptides as a decontamination agent to treat stomach ten. New drugs for duodenal ulcers and gastrointestinal tract inflammation are under way. The frog has many glands on its skin and can produce many kinds of biologically active peptides. The Magainin antibacterial peptides have been genetically engineered to synthesize new Magainin drugs. 3.3 Genetic engineering of silkworm antibacterial peptides Antimicrobial peptides have a broad-spectrum antibacterial activity, but their contents are low in normal animals, making them difficult to apply. The antibacterial peptides developed and applied so far are all extracted from immune cocoons, and the content of antibacterial peptides in immune serum of tussah cocoon is very low, and the extraction price is high. In order to reduce the production cost of antibacterial peptides, large-scale industrial production has been implemented. Artificially designed and synthesized antimicrobial peptides D, BD and AD genes, and successfully introduced into Saccharomyces cerevisiae and Pichia pastoris for expression. 4 Advantages of Antimicrobial Peptides Used as Feed Additives Antibacterial peptides can tolerate high temperatures during feed granulation. Large-scale production of antimicrobial peptides through the high-temperature concentration process can fully kill the yeast cells without inactivating antibacterial peptides. After the product is popularized and applied, there will be no proliferation of engineering bacteria and environmental ecological problems. Antimicrobial peptides have unique bactericidal mechanism, pathogenic bacteria are not resistant to antimicrobial peptides, and industrial bacteria produce industrial antimicrobials to produce antibacterial peptides. They can be produced on a large scale, have a short production cycle, have a low production cost and are not affected by the external environment such as seasonality and climate change. . In addition, the antibacterial peptide product has a broad-spectrum antibacterial effect, and has the functions of promoting growth, health care, and treating diseases for livestock and poultry, and is an environment-friendly agent having no toxic side effects, no residues, and no bacterial drug resistance. In short, antimicrobial peptides have a unique antibacterial mechanism that is different from antibiotics. It is widely found in a variety of organisms and is the product of a series of immune responses produced by organisms infecting external pathogens. It has a strong broad-spectrum antimicrobial capacity. . The search for endogenous antibacterial substances in various animals and the industrial production of these products with animal specificity or specificity on pathogen-resistant bacteria using modern biotechnology methods are bound to become novel feed additives that replace antibiotics.