Antimicrobial peptides (AMPs) are a class of small peptides that widely exist in nature and they are an important part of the innate immune system of different organisms. So far, more than 2,600 antimicrobial peptides have been discovered. They are widely distributed, including viruses, bacteria, fungi, fish, birds, insects, amphibians, mollusks, and mammals.
Sources of Antimicrobial Peptides
- Microorganisms-Derived Antimicrobial Peptides
Microorganisms-derived antimicrobial peptides are classified into two types: antimicrobial peptides derived from bacterial and antimicrobial peptides derived from viral. Among them, antibacterial peptides derived from bacteria are also called bacteriocins. Bacteriocins are a type of peptide or precursor peptide molecules with bactericidal biological activity that are produced by bacteria during the metabolic process through protein. Usually, these peptides are all cationic amino acid chains, which are hydrophobic or amphiphilic. According to biochemical properties, bacteriocins can be divided into three categories: class I bacteriocins, class II bacteriocins and class III bacteriocins. Among them, type I bacteriocins are also called thioether antibiotics, with a molecular mass of less than 5ku and a number of amino acids of about 19 to 38. They have mucosal activity and good heat resistance. Their structure contains rare thioether amino acids, such as Lanthionine and β-methyllanthione. Class II bacteriocins are a type of antibacterial peptides called lactic acid bacteriocins discovered in recent years. Based on their biological, physical and chemical properties, they can be used as food preservation agents. Compared with class I bacteriocins, class II bacteriocins have a slightly smaller molecular mass (4-6 ku). Class III bacteriocins are secreted by bacterial proprotein translocases, with a molecular weight greater than 30 ku and good heat resistance. The other type is antibacterial peptides derived from viral, which are currently found in relatively few types. Among them, LLPs are lentiviral cytolytic peptides encoded by the C-terminal sequence of the human defect virus type 1 (HIV-1) transmembrane protein, which have strong toxic effects on microorganisms and cells. LLPs have high arginine and do not contain lysine. In most cases, microorganisms-derived antimicrobial peptides exist in a cyclic structure.
- Plant-Derived Antimicrobial Peptides
Plant-derived antibacterial peptides have active effects on plant pathogens and bacterial pathogens that infect humans. They are a barrier to the plant defense system and considered to be antibiotic peptides with important application. Common plant-derived antibacterial peptides include thionin, defensins, lipid transfer proteins, cell penetrating peptides, ecdysone, etc. They have similar characteristics, such as positive charge, disulfide bond, and the target sites are all located in the outer cell membrane and so on.
- Insect-Derived Antimicrobial Peptides
Insects are the most abundant creatures in the biological world, with more than 1 million species. They also have strong adaptability and developed defense mechanisms. Antimicrobial peptides are a type of peptide produced by the hemolymph or related parts of the body when insects are stimulated by the external environment. They are an organic part of the insect’s humoral immune system and play an important role in the entire immune system. Antibacterial peptides secreted by its fat body inhibit pathogens in various systems. So far, there are about 200 kinds of antibacterial peptides isolated in insects. These peptides are divided into 5 types according to their antibacterial mechanism and amino acid sequence: cecropins, lysozyme, defensins, glycine-rich peptides and proline-rich peptides.
- Amphibian-Derived Antimicrobial Peptides
The skin of amphibians is exposed, the humidity is high, and it has respiratory function. This morphological and physiological characteristic requires that it can only survive in a humid environment, which is also an excellent environment for some pathogenic microorganisms to survive. In order to exploit and adapt to vast habitats and diverse ecological environments, amphibians eventually formed a defense system against pathogenic microorganisms through continuous evolution. Antibacterial peptides, an important effector molecule of innate immunity released by skin glands, play an important role in the immune defense mechanism of amphibians when they are stimulated by adrenaline, stress, trauma and other factors. After long-term evolution, the antimicrobial peptide gene has undergone multiple duplications and mutations, resulting in a large number of structures and a wide variety of antimicrobial peptides. At present, 1,400 antimicrobial peptides have been isolated from amphibian skin secretions, and secretions from different sources contain different antimicrobial bioactive molecules. Amphibian-derived antimicrobial peptides can be divided into two categories according to their structural characteristics: cyclic antimicrobial peptides containing intramolecular disulfide bonds and linear antimicrobial peptides with α-helical structure.
- Mammalian Antimicrobial Peptides
Antimicrobial peptides in mammals only have a lethal effect on exogenous bacteria, but are harmless to normal cells of the animal. Mammalian antimicrobial peptides mainly exist in mucosal epithelial cells and neutrophils and skin, such as surfactant anionic antimicrobial peptides in the respiratory tract, and defensins in epithelial tissues. These antimicrobial peptides and phagocytes jointly establish the body’s first line of defense. Mammalian antimicrobial peptides are mainly divided into two categories, namely defensins and cathelicidins. The most studied defensin antimicrobial peptides are the largest class of antimicrobial peptides. Mammalian defensins are divided into two categories: α-defensins and β-defensins.
- Marine Antimicrobial Peptides
The ocean is the origin of life and a huge living place for creatures. It is estimated that about 1/2 of the world’s creatures live in the ocean. This huge biobank contains abundant active antibacterial substances. It has been proved before that a wide variety of marine organisms can produce a variety of antibacterial peptides, of which the research on marine animal antibacterial peptides accounts for the majority. Among marine vertebrates, fishes are the most reported to secrete antimicrobial peptides. Due to the specificity of its body structure, antimicrobial peptides are mainly secreted into the mucus layer of marine fish, which act as the first barrier of the non-specific immune system to kill pathogenic bacteria before entering the organism. Marine biological antimicrobial peptides have the characteristics of rapid synthesis and secretion, fast diffusion and flexibility, which are beyond the functions of other immune system. Marine invertebrates such as molluscs, tunicates, crustaceans, etc. mainly rely on the innate immune mechanism of humoral immunity and cellular responses as defense systems. Among them, antimicrobial peptides, as an important component of the innate immune defense system, can respond quickly and in real time to invading pathogenic microorganisms.