(1) Insect antimicrobial peptides: Insects constitute the largest group of organisms, and the number of antimicrobial peptides they produce is immense. To date, over 200 types of insect antimicrobial peptides have been identified across eight orders, including Lepidoptera, Diptera, Coleoptera, and Odonata; notably, 40 antimicrobial peptide genes have been isolated from the silkworm (*Bombyx mori*) alone.
(2) Mammalian antimicrobial peptides: The mammalian antimicrobial peptide Cecropin P1 was first isolated from the small intestine of a pig in 1989. Currently, at least 18 types have been isolated from pigs, 30 from sheep, and 30 from cattle.
(3) Amphibian antimicrobial peptides: Amphibians possess naked skin that serves multiple functions. Their skin secretions contain a vast array of bioactive peptides with diverse biological activities; most of these exhibit antimicrobial properties. Representing an ancient and effective form of natural defense, they are generally classified as antimicrobial peptides.
(4) Antimicrobial peptides from fish, mollusks, and crustaceans: Pardaxin, a 35-amino-acid-residue peptide isolated from the Moses sole (*Pardachirus marmoratus*) in 1986, was the first antimicrobial peptide isolated from fish to feature an amphipathic, cationic α-helical structure capable of membrane permeabilization. It functions as an ionic neurotoxin, and a series of derivative peptides have been developed that exhibit stronger antimicrobial activity-and lower hemolytic activity-than melittin.
(5) Plant antimicrobial peptides: Plants also produce antimicrobial peptides structurally similar to the defensins found in insects and mammals; these are known as plant defensins. Most plant antimicrobial peptides are highly active against plant pathogens, though some also exhibit toxicity toward Gram-positive and Gram-negative bacteria, fungi, yeasts, and mammalian cells. Thionins were the first antimicrobial peptides to be isolated from plants.
(6) Bacterial antimicrobial peptides: Also known as bacteriocins, these include both cationic and neutral peptides and are secreted by both Gram-positive and Gram-negative bacteria. Four types of antimicrobial peptides have been identified in bacteria: bacitracin, gramicidin S, polymyxin E, and nisin.
