Antibacterial peptides (AMPs) are protein fragments that have an antimicrobial activity. They are produced by bacteria or part of the bacterial cell wall or membrane structure, and have a wide range of functions. They can be classified as antibacterial, antiviral, antifungal, and antiparasitic peptides.
AMPs can be isolated from several sources including bacteria, fungi, yeasts and a wide variety of other organisms (Figure 2). They have different targets, which include proteins, lipids, enzymes and other biomolecules that are important for cell function. Amongst the major targets for AMPs are proteins, lipids and mannoproteins from microbial cells and extracellular polymeric substances (EPS) from the bacterial cell wall.
In addition, AMPs can inhibit host cell proteases and microbial serine, arginine, or lysine proteases, which is an important property in defending against microbes . Some AMPs also have an inhibitory effect on phagocytosis. This mechanism is important in neutrophils and cytotoxic T lymphocytes, which are specialized for phagocytosis.
Another method for increasing AMPs’ activities is to add post-translational modifications to the peptide chain, such as amidation or acetylation at the N-terminus or C-terminus, which can produce more natural peptide characteristics (Datta et al., 2016). Some AMPs are N-terminally lipidated. Lipidation can increase the neutralization of LPS by lipid rafts, increase the stability of the AMPs to proteases and peptidases, and reduce the toxicity of the AMPs towards cells.
Glycine-rich AMPs are often used to enhance antibacterial activity against Gram-negative bacteria, due to their non-polar nature and the effect of glycine residues on the tertiary structure of the peptide chain. Salmonid cathelicidins, for instance, can activate phagocyte-mediated microbicidal mechanisms and are considered ideal antibacterial peptides against Gram-negative bacteria.