To date, more than 200 antimicrobial peptides have been isolated from various organisms, with over 170 derived from insects alone. These peptides are classified based on their origins and structural properties.
Linear peptides with a helical structure
Cecropins were the first animal antimicrobial peptides discovered; they were isolated from pupae of the American silkmoth (*Hyalophora cecropia*) by Boman et al. in 1980.
Linear peptides rich in specific amino acids
Apidaecins are proline-rich peptide antibiotics isolated from honeybees. They typically consist of 16–18 amino acid residues, with proline content reaching up to 33% and arginine content up to 17%. Apidaecins exhibit strong activity against certain Gram-negative bacteria but are ineffective against Gram-positive bacteria.
Peptides containing a single disulfide bond
This is a small class of antimicrobial peptides. The first peptide of this type discovered was bactenecin, derived from bovine neutrophils. It contains 12 amino acids-including four arginine residues-and forms a disulfide bond between the second and eleventh residues.
Peptides containing two or more disulfide bonds
Defensins are the prototypical representatives of this class. The initially discovered α-defensins, derived from mammalian tissues, generally contain 29–34 amino acid residues; six conserved cysteine residues form three intramolecular disulfide bonds. Additionally, arginine residues at positions 6 and 15, as well as a glycine residue at position 24, are conserved.
Lantibiotics
Lantibiotics are peptide antibiotics produced by bacteria; they are genetically encoded, synthesized on ribosomes, and undergo post-translational modification to incorporate specific organic groups. Nisin is the most extensively studied among them.
