Antimicrobial peptides (AMPs) are small, naturally occurring peptides found in multicellular organisms as the first line of defense against microorganisms including bacteria, yeasts, fungi and viruses. They are synthesized by cells of the innate immune system and have a broad spectrum of activity that can inhibit or kill many types of pathogenic and commensal microorganisms.
The broad antibacterial activity of AMPs is mainly due to the disruption of cell membrane. For example, ciprofloxacin (a synthetic antibiotic) and tetracycline (a widely used antimicrobial) both have similar antibacterial activities by disrupting the outer layer of the cell membrane.
Other AMPs have a strong activity by targeting specific intracellular structures. For example, hevein-like peptides derived from Bungarus fasciatus venom act against Gram-positive bacteria, by binding to ribosomal proteins and inhibiting protein synthesis. Similarly, circulin-like peptides from cows and chickens bind to the bacterial lipopolysaccharide phospholipid A and block its permeability. Defensin-like peptides from black sea bass fish, such as cerocin and jelleine-I, also have antibacterial activities, by binding to the peptidoglycan residues in the bacterial cell walls and blocking protease activities (Kuhl et al., 2015).
The mechanism of action of AMPs depends on the sequence and structural conformation of their amino acid fragments. For example, AMPs with a b-sheet structure show stronger activity, because they can assemble in the Barrel-stave model, where the positively-charged peptide residues interact with the head groups of phosphates from the membrane, leading to pore formation and membrane disintegration.