Insect antimicrobial peptides, as the earliest discovered antimicrobial peptides, have the advantages of small molecular weight, no immunogenicity, strong thermal stability, broad-spectrum sterilization and so on. It has shown broad prospects in agriculture, livestock and poultry breeding, medicine and food industry, and is used to cultivate new crop varieties resistant to pests and diseases, replacing antibiotics as new non-toxic and pollution-free antibacterial agents as feed additives. Key words: insect antimicrobial peptides; Structure; Antibacterial mechanism; Antimicrobial peptides are small peptides with biological activity induced by organisms, with a molecular weight of about 2000 ~ 7000 and composed of 20 ~ 60 amino acid residues. Most of these active peptides have strong alkalinity, thermal stability and broad-spectrum antibacterial properties, and more than 600 kinds have been found in insects, birds, animals, plants and prokaryotes. 1 Overview of Insect Antimicrobial Peptides Insects are numerous and widely distributed, with high adaptability and defense mechanism. 1972, Swedish scientist Boman and others first isolated the first efficient antibacterial peptide Cecropins from the immune hemolymph of silkworm chrysalis, and then isolated antibacterial peptides from silkworm, tussah, castor silkworm and other insects. At present, 170 kinds of insect antimicrobial peptides have been found. Studies have shown that insect antibacterial peptides have broad-spectrum antibacterial activity, have obvious killing effect on drug-resistant strains, and are not destructive and immunogenic to biological cells. Their production and release are part of the body's inflammatory reaction and an important barrier for the host to defend against the invasion of pathogenic microorganisms such as bacteria and fungi, and their application prospects are very broad. Although the molecular size and amino acid composition of insect antimicrobial peptides are different, their common characteristics are quite obvious. Generally, their molecular weight is less than 5000, and they are basic peptides, and most of them form alpha helix structure and beta lamellar structure or their mixed structure. Insect antimicrobial peptides have been studied by scientists for many years, and they are usually divided into four categories according to their composition and structural characteristics. 2. 1 cecropin, represented by cecropin, consists of 3 1 ~ 39 amino acid residues. It is an amphiphilic α -helix antibacterial peptide, and contains no cysteine and disulfide bonds. N-terminal region is strongly alkaline, half of C-terminal region is neutral hydrophobic region, and its isoelectric point is 8.9 ~ 9.5. These antimicrobial peptides are widely found in insects, and more than 20 antimicrobial peptide analogues have been isolated from Lepidoptera and Diptera. 2.2 Insect defensins Most defensins are composed of 38 ~ 43 amino acid residues, forming an amphipathic α -helix structure in the molecule, containing 6 Cys in the molecule, which can form three intramolecular disulfide bonds and form a β -sheet structure, in which disulfide bonds are closely related to antibacterial activity. 2.3 Insect antimicrobial peptides rich in proline residues These antimicrobial peptides are isolated from Diptera, Hymenoptera, Hemiptera and Coleoptera. They have high similarity, containing 15 ~ 39 amino acid residues, and the proline content in the molecule is more than 25%, which is similar to some mammalian antibacterial peptides, among which apidaecins is a proline-rich antibacterial peptide found in HYMENOPTERA. Including 17 species, has strong activity on gram-negative bacteria, but has no effect on gram-positive bacteria. Apidaecins has a high mortality rate for some Gram-negative plant pathogens and Enterobacteriaceae pathogens, which makes it have a good application prospect in plant antibacterial genetic engineering and food industry. 2.4 Antimicrobial peptides rich in glycine, similar in structure to apidaecins, have been isolated from Diptera, Lepidoptera, Hymenoptera, Coleoptera and other insects. The primary structure of these antimicrobial peptides is rich in glycine, and the molecular size is 8000 ~ 30000. It is speculated that this antibacterial peptide with high content of G 1y may improve the elasticity of peptide chain and broad-spectrum antibacterial activity. 3. Mechanism of Action of Insect Antimicrobial Peptides 3. 1 Cell Membrane Potential Dependent Channels Most of the amino acids of antimicrobial peptides are positively charged, and the molecules form electrostatic adsorption with the negative charges on the phospholipid molecules in bacterial cytoplasm, and then the hydrophobic end of the molecules is inserted into the plasma membrane by virtue of the flexibility of the molecular chain, thus dragging the whole molecule into the plasma membrane, destroying the original arrangement order of protein and lipids on the plasma membrane. Then the transmembrane ion channel is formed by mutual substitution of antibacterial peptide molecules. After the transmembrane ion channel is formed on the plasma membrane, a large number of ions in the cell are lost, and finally the cell cannot maintain its normal osmotic pressure and dies. 3.2 Theory of inhibitory effect of antimicrobial peptides on cell respiration Fehlbaum et al. (1996) found that when the concentration of insect defensin thanatin was 0.3 ~ 0.6 μ mol/L, it showed a strong bactericidal effect on Escherichia coli, but when its concentration increased to 70μmol/L, the leakage of intracellular potassium ions could not be detected. This shows that the death element is not sterilized by changing the permeability of cell membrane, but when bacterial lh is treated with 40μmol/L death element, the respiration of cells becomes weak and stops completely after 6 hours, so it is inferred that death element is sterilized by inhibiting the respiration of cells. 3.3 Other Mechanisms of Action 3.3. 1 Inhibition of the Synthesis of Cell Outer Membrane Proteins Carsson et al. (199 1 year) found that attacin can interfere with the transcription of the outer membrane proteins Omp C, Omp F, Omp A and LamB of E.coli cells, and reduce the contents of these proteins, thus leading to the increase of cell membrane permeability and the inhibition of bacterial growth. 3.3.2 Inhibition of cell wall formation Ando et al. (1988) believe that myotoxin II can inhibit the formation of bacterial cell walls, so that bacteria can not maintain normal cell morphology and their growth is hindered, but it has no effect on the already formed cell walls. In addition, some scientists believe that antibacterial peptides containing proline combine with bacterial heat shock proteins to play an antibacterial role; Another study shows that antibacterial peptides can achieve bactericidal function through the synergistic effect of many factors; Antibacterial peptides can also improve antibacterial ability by promoting the immune function of the body. 4 Application of Insect Antimicrobial Peptides Antimicrobial peptides have broad-spectrum antibacterial effects, and have the functions of promoting growth, health care and treating diseases for livestock and poultry. They are a kind of environment-friendly preparations with no toxic and side effects, no residue and no bacterial resistance, but their content in normal animals is low, which makes their application difficult. Usually, the antibacterial peptides developed and applied are extracted from the immunized silkworm pupae, but the content of antibacterial peptides in the immune serum of tussah pupae is very low and the extraction price is expensive. After research, it is possible to transfer antimicrobial peptide genes through bioengineering technology to produce transgenic animal and plant products with antibacterial activity. At the same time, the antibacterial peptide can be expressed in large quantities through genetic engineering technology, making it a source of a new generation of peptide antibacterial drugs. Its application in food industry mainly focuses on the production of food raw materials. 4. 1 application in agriculture The application of insect antimicrobial peptides in agriculture is mainly used to cultivate crops with pathogen-resistant varieties, so as to minimize the food safety problems caused by the use of pesticides in the growth of food raw materials. In addition, the antimicrobial peptide gene was transferred into plants by transgenic engineering to obtain disease-resistant varieties, which found a new way to obtain disease-resistant varieties. 4.2 Application in Livestock and Poultry Breeding Insect antimicrobial peptides are considered as the most promising green feed additives. Antibiotics and antibiotic additives are widely used in livestock and poultry breeding, which not only destroy the microbial balance of animal intestines, but also remain in animals, seriously affecting the quality of livestock products and adversely affecting human health. Antibiotics as feed additives will seriously affect the competitiveness of livestock and poultry products in the international market. Experiments show that antibacterial peptides can promote the growth of poultry, reduce the nitrogen content in excreta, and have the functions of promoting growth, health care and treating diseases for poultry. The application effect of silkworm antibacterial peptide yeast preparation as feed additive is remarkable. At the same time, using genetic engineering technology, through the study of antimicrobial peptides of livestock and poultry, using modern biotechnology methods to transfer specific antimicrobial peptide genes into specific cells of livestock and poultry, and express their characteristics of killing harmful bacteria in livestock and poultry, so as to produce new disease-resistant varieties, improve the disease-resistant ability of livestock and poultry, and reduce or even replace the use of antibiotics, which will play a positive role in promoting the development of animal husbandry production and have broad application prospects. 4.3 Development of New Anti-infective Drugs Antibacterial peptides have been used in the clinical application of cerebrospinal meningitis, Helicobacter pylori infection and anti-fungal infection. The medicinal value of insect antimicrobial peptides has long been recorded in traditional Chinese medicine. After cleaning and drying, housefly larvae are mixed with other traditional Chinese medicines to prepare medicines for treating infectious diseases. With the development of genetic engineering technology, the production of insect antimicrobial peptides has begun to be commercialized, and will be more widely used in agricultural, industrial and food production fields. However, how to improve the production efficiency, reduce the cost and further improve the antibacterial activity of insect antibacterial peptides still needs further study and discussion. Spirulina is a lower plant, belonging to Cyanophyta and Trematodaceae. Like bacteria, they have no real nuclei in their cells, so they are also called cyanobacteria. Cyanobacteria, with primitive cell structure and very simple structure, is the earliest photosynthetic organism on earth, which has existed on this planet for 3.5 billion years. It grows in water, and it can be seen under the microscope that it is a spiral filament, hence its name. Spirulina is a specialty of Yunnan, and Chenghai Lake is the only place where Spirulina can be cultivated in China, and the growth environment of Spirulina is also very demanding. In China, basically all spirulina is cultivated artificially, and the quality of production is also uneven. The broken-wall fishy spirulina sold by merchants is only a concept. Spirulina does not need to be broken, and its outer wall is not made of fibrous tissue, which is easy to be digested and absorbed by human body. It is impossible to remove fishy smell simply by physical and mechanical means. Only when other fishy substances are added to spirulina, spirulina is not pure spirulina as mentioned by merchants. Spirulina is a microorganism growing in alkaline environment, and it is the most abundant and balanced natural food discovered by human beings so far. It is rich in protein, vitamins (B 1, B2, B6, B 12, VE, carotene and α-linolenic acid), minerals (calcium, iron, zinc and selenium), especially iron, and eight essential amino acids. Spirulina is high in protein, low in fat and rich in nutrition, which has the functions of prevention and health care for many diseases, can effectively reduce blood sugar and blood lipid, protect liver, resist oxidation, resist radiation, resist tumor and improve immunity, and has no negative impact on human body. Spirulina is internationally recognized as the single most effective anti-radiation substance. The nutritious meal is made of high-quality spirulina, which is instantly dried by international advanced equipment to ensure that the active substances in spirulina are intact.