Antimicrobial peptides, also known as antimicrobial peptides or peptide antibiotics, are widely distributed in animals and plants and are part of the natural immune defense system. Antimicrobial peptides not only have broad-spectrum antibacterial activity, but also have effects on fungi, viruses and cancer cells. In recent years, problems such as drug abuse, drug residues and bacterial resistance have become increasingly serious. This has led to people’s attention to food and the environment. More and more countries are calling for the ban on antibiotics, and antimicrobial peptides are The activity and the special mechanism of action that is different from traditional antibiotics have caused great research interest and become one of the hot spots in the field of molecular biology and biochemistry research. The antibacterial peptide was first induced by the Swedish scientist Boman G et al. to induce the antibacterial activity of the polypeptide, Cecropin, produced by the ciprofloxacin and the Escherichia coli. Since then, scientists have discovered and isolated more than 300 endogenous antimicrobial peptides in insects, tunicates, amphibians, birds, fish, mammals, plants, and even humans. Recently, more systematic theoretical and applied research has been conducted on insect antimicrobial peptides, and genetic engineering peptides have been used for experimental production. 1 Structure and characteristics of antimicrobial peptides 2 antimicrobial mechanism and characteristics of antimicrobial peptides 2.1 Mechanism of action of antimicrobial peptides Studies have shown that antibacterial peptide molecules aggregate with each other through intermolecular displacements in the membrane, thereby forming ion channels on the membrane, causing membrane proteins to agglutinate and preventing bacteria from maintaining normal osmotic pressure. death. It has also been proposed to kill bacteria by affecting the energy transport and metabolism on the cell membrane, thereby damaging the cellular respiratory chain. Antibacterial peptides can also break the nuclear DNA of cancer cells and kill cancer cells by inhibiting DNA synthesis. In summary, the key to the mechanism of action of antibacterial peptides lies in their physical and cell membrane effects. The mechanism of action of different classes of antibacterial peptides may be different. 2.2 Characteristics of Action Mechanism of Antibacterial Peptides 3 Biological characteristics of antimicrobial peptides 3.2 Fast, safe, and non-residual effects Since the molecule is small, it can be generated within a few minutes when the body is invaded, and the expanded diffusion speed is faster and more flexible than the immune cells in the body. The unique antibacterial mechanism of antibacterial peptides has formed a non-resistance characteristic, which surpasses the limitations of antibiotics and is a new generation of highly effective antibacterial agents. Animal food safety depends on various aspects of the production process. The use of non-pollution additives that have no residues in the animal body and no toxic or side effects is a key to solving the problem. Antimicrobial peptides are one of the animal body components and participate in the life process. They are small, short peptides and have the biological characteristics of safe, non-toxic side effects. 3.3 Stimulation of immune responses Antimicrobial peptides may participate in other reactions in the host's natural immunity, such as stimulating the chemotaxis of monocytes and neutrophils, promoting the release of histamine in mast cells, inhibiting cathepsins, and promoting wound healing. 3.4 The joint action study with other antibacterial substances found that antibacterial peptides combined with traditional antibiotics can increase the efficacy and broaden the antibacterial spectrum of traditional antibiotics. There is also synergy with lysozyme. So far, no reports of antagonism between antimicrobial peptides and other drugs have been reported. This shows that antimicrobial peptides are more advantageous than green additives such as microecological preparations. 3.5 It has strong resistance to the environment, and it is suitable for the additive antimicrobial peptide to keep vigor even when heated at 100°C for 10 minutes. It has strong adaptability to pH and good water solubility. Some antibacterial peptides still have the ability to resist the hydrolysis of trypsin or pepsin, enter the digestive tract of animals, and can exert antibacterial effects. This makes up for the susceptibility of other additives such as enzyme preparations, microecological preparations and the like, and is more convenient for use in production. 4 Application of antimicrobial peptides 4.2 Application in animal husbandry Diarrhea in piglets, mastitis in cows 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 the successful transgenic insect antibacterial peptide engineering, such as genetically modified mosquitoes, transgenic potatoes, transgenic rice, etc., and transfer specific antibacterial peptide genes to specific cells of livestock and poultry for expression, resulting in new disease-resistant varieties. 5 Application Prospects 5.1 The ideal alternative to antibiotics At present, all the conventional antibiotics have appeared the corresponding resistant disease-causing strains. The resistance of pathogenic bacteria has increasingly threatened people's health. Finding new types of antibiotics is an effective way to solve the problem of drug resistance. Antimicrobial peptides are considered to have broad application prospects because of their high antibacterial activity, broad antibacterial spectrum, variety of species, wide range of available options, and difficulty in producing resistance mutations in target strains. More importantly, peptides are diverse in sequence and structure, providing a lot of imagination and space for new drug design, and can produce stable products. 5.2 The potential for development and application The current genetic engineering of insect and plant antimicrobial peptides has been successfully reported at home and abroad, but the genetic engineering of livestock and poultry antimicrobial peptides has not been reported. Therefore, the use of genetic engineering techniques to improve the disease resistance of livestock and poultry and aquaculture animals through research on livestock and poultry and aquatic animal antibacterial peptides will play a positive role in reducing or even replacing the use of antibiotics. With the development of molecular biology technology, the application prospect of antimicrobial peptides in animal production will be very broad. The search for a variety of animal endogenous antibacterial substances, industrial production of these animal-specific series of products against pathogenic bacteria using modern biotechnology methods will undoubtedly be another way out for antimicrobial peptides. 6 Problems 6.2 Compared with traditional antibiotics, the antibacterial activity of insect antibacterial peptides is still not ideal. The transformation of existing antibacterial peptides and the design of new antibacterial peptide molecules are effective ways to create highly active antibacterial peptides. This requires the further study of the relationship between the structure and activity of antimicrobial peptides and the mechanism of action, providing sufficient theoretical basis for the modification and design of antimicrobial peptides. 6.3 Animals have different tolerance levels for various peptides. This issue needs further study.
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Natural antibacterial peptides are usually basic small molecule polypeptides composed of more than 30 amino acid residues. They have good water solubility and a molecular weight of approximately 4 kDa. Most of the antimicrobial peptides have a thermal stability, and their activity can still be maintained by heating at 100° C. for 10-15 min. The isoelectric point of most antimicrobial peptides is greater than 7, showing strong cationic characteristics. The N-terminus of the amino acid is rich in cationic amino acids such as lysine and arginine, and its C-terminus is rich in apolar amino acids such as alanine, proline, and glycine. There are some more conservative amino acid residues in many specific positions. These highly conserved amino acid residues are indispensable for antibacterial activity of some antibacterial peptide molecules. In addition, the C terminus of some natural antibacterial peptides is often amidated. Antimicrobial peptides are associated with a broad spectrum of antimicrobial activity. Antimicrobial peptides form α-helices and β-sheet structures under certain conditions.
2.2.1 Effectiveness of Action Sites The role of traditional antibiotics is achieved by eliminating the growth of microorganisms or the necessary functions for survival, such as blocking the synthesis of bacterial proteins or changing the activity of enzymes to achieve the purpose of bactericidal, and bacteria by changing a gene Enough to deal with this attack of antibiotics. Antimicrobial peptides act on bacterial cell membranes, resulting in increased membrane permeability, which penetrates and kills bacteria. Bacteria must change the structure of the membrane, that is, change a considerable part of the gene to defend against the attack of antimicrobial peptides, which is almost impossible. Therefore, antimicrobial peptides greatly reduce the possibility of drug resistance.
2.2.2 Selective antibacterial peptides that act on the target will only have antibacterial effects on prokaryotic cells and eukaryotic lesions and will not function on normal eukaryotic cells. The reason is that the structure of the cell membranes of prokaryotes and normal eukaryotes is different, and normal eukaryotic cell membranes contain a large amount of cholesterol, and the presence of cholesterol stabilizes the membrane structure. In addition, there are highly developed cytoskeletal systems in higher animals, and their presence is also resistant to the action of antimicrobial peptides. The cytoskeletal system of cancer cells is less developed than normal cells, which is one of the reasons that antimicrobial peptides inhibit it.
3.1 Broad-spectrum anti-microbial The current livestock and poultry diseases are mostly mixed infections. The antibiotic spectrum of traditional antibiotics is generally narrow, and it is only effective against bacteria. Most antimicrobial peptides are effective against Gram-negative bacteria, and some also have inhibitory or killing effects on fungi, viruses, parasites, and protozoa. Experiments have shown that antimicrobial peptides can kill paramecium, amoeba, and tetrahymena. Tussica antibacterial peptide D has killing effect on Trichomonas vaginalis.
Many studies have shown that certain types of antimicrobial peptides have anti-DNA and RNA viruses. Melittins (metoxin) and Cecropin inhibit HIV-1 virus proliferation by suppressing gene expression at sub-toxic concentrations. Magainin-2 and synthetic peptides Modelin-1 and Moderln-5 had certain inhibitory effects on herpes virus HSV-1 and HSV-2. Wachinger M et al. reported that cecropin can inhibit HIV. The silkworm antibacterial peptide AD developed by South China Agricultural University had inhibitory effects on the DNA proliferation of duck hepatitis B.
4.1 Production of Antimicrobial Peptides The most studied now is the insect antibacterial peptide. Although it can be extracted from the body, its content is extremely small, which brings great difficulties to its application. The antibacterial peptides studied so far are all extracted from the immune cocoon, and the antibacterial peptide content in the immune serum of tussah silkworm is very low, and the extraction price is expensive. In order to reduce the production cost of antibacterial peptides and carry out large-scale industrial production, antibacterial peptides D, BD and AD genes have been artificially designed and synthesized, and successfully introduced into Saccharomyces cerevisiae and Pichia pastoris for expression. Because antimicrobial peptides have a strong killing effect on bacteria, they are not easily expressed directly in the prokaryotic system and must be expressed by yeast. Expressions obtained in yeast include: bombesin (Chen Haixu et al., 2002), antimicrobial peptides AD (Huang Yadong et al., 2002), antimicrobial peptides Apila (Han Wanjiang et al., 1998), and antimicrobial peptides ABP3 (Shen Junqing et al., 1999).
Wen Liufa et al. Applied antibacterial peptides that have been developed in South China Agricultural University were added to feed for weaned piglets. Feeding test results showed that silkworm antibacterial peptides can reduce diarrhea in weaned piglets. Chen Xiaosheng et al. reported that the addition of antimicrobial peptides to diets promotes growth of meat ducks. It was also reported that tussah silkworm antibacterial peptides are effective in preventing and treating chickens. Huang Yongzheng et al used silkworm antibacterial peptide AD-yeast preparation with 5 kinds of antibiotics and 3 kinds of Chinese herbal medicines for broiler feeding comparison test. The results showed that there was no difference in feed conversion rate, average body weight, feed ratio, and survival rate. It is highly desirable to improve the quality of livestock products and ensure the production efficiency of livestock and poultry products.
Wen Liufa et al. used an experimental yeast fermentation broth as a test. The results of the feeding trials of Guangdong yellow chicken that ingested a certain dose of antimicrobial peptides by drinking water showed that the antimicrobial peptides can promote chick growth and reduce fecal nitrogen content. The function of growing, health care and treating diseases. The application effect of the silkworm antibacterial peptide yeast preparation as feed additive is significant. The domestic chicken king Guo Xinxin used artificially reared fly maggots to raise chickens, with large egg yolks, low cholesterol content, few chickens to get sick, and good economic returns. It has established a chain breeding base in various parts of the country. It is worth investigating and vigorously promoting the natural use of antimicrobial peptides in insects. Nankai University, Tianjin University, and Dagang Oilfield have jointly tackled the problem and successfully isolated antibacterial peptides that inhibit multiple pathogenic bacteria and viruses from the flies. Currently, a number of antibacterial tests have been completed, and researchers are proceeding with further purification to extract them from fly larvae. Antibacterial peptides. This shows that antimicrobial peptides and related industries will play an important role in animal husbandry.
In addition, according to American scholars, antimicrobial peptides can also be used as feed fungicide.
6.1 The natural resources of antimicrobial peptides are limited. Chemical synthesis and genetic engineering have become the main means for obtaining antimicrobial peptides. Chemically synthesized peptides are costly. However, through genetic engineering, the direct expression of antimicrobial peptide genes in microorganisms may cause host microorganisms to commit suicide without obtaining expression products. The expression of the antibacterial peptide gene in the form of a fusion protein can overcome this disadvantage, but it still has the problem of less expression product. 