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Antimicrobial peptides (also called host defence peptides) are an evolutionarily conserved component of the innate immune response and are found among all classes of life.
These peptides are potent, broad spectrum antibiotics which demonstrate potential as novel therapeutic agents. Antimicrobial peptides have been demonstrated to kill Gram negative and Gram positive bacteria (including strains that are resistant to conventional antibiotics), mycobacteria (including Mycobacterium tuberculosis), enveloped viruses, fungi and even transformed or cancerous cells. Unlike the majority of conventional antibiotics it appears as though antimicrobial peptides may also have the ability to enhance immunity by functioning as immunomodulators.
Additional recommended knowledge
Antimicrobial peptides are short proteins, generally between 12 and 50 amino acids long (although larger proteins with similar properties such as lysozyme are often classified as antimicrobial peptides). These peptides include two or more positively charged residues provided by arginine, lysine or, in acidic environments, histidine, and a large proportion (generally >50%) of hydrophobic residues. The secondary structures of these molecules follow 4 themes, including i) α-helical, ii) β-stranded due to the presence of 2 or more disulphide bonds, iii) β-hairpin or loop due to the presence of a single disulphide bond and/or cyclization of the peptide chain, and iv) extended. Many of these peptides are unstructured in free solution, and fold into their final configuration upon partitioning into biological membranes. The ability to associate with membranes is a definitive feature of antimicrobial peptides although membrane permeabilisation is not necessary. These peptides have a variety of antimicrobial activities ranging from membrane permeabilization to action on a range of cytoplasmic targets.
The modes of action by which antimicrobial peptides kill bacteria is varied and includes disrupting membranes, interfering with metabolism, and targeting cytoplasmic components. In many cases the exact mechanism of killing is not known. In contrast to many conventional antibiotics these peptides appear to be bacteriocidal (bacteria killer) instead of bacteriostatic (bacteria growth inhibitor). In general the antimicrobial activity of these peptides is determined by measuring the minimal inhibitory concentration (MIC), which is the lowest concentration of drug that reduces growth by more than 50%.
In addition to killing bacteria directly they have been demonstrated to have a number of immunomodulatory functions that may be involved in the clearance of infection, including the ability to alter host gene expression, act as chemokines and/or induce chemokine production, inhibiting lipopolysaccharide induced pro-inflammatory cytokine production, promoting wound healing, and modulating the responses of dendritic cells and cells of the adaptive immune response. Animal models indicate that host defence peptides are crucial for both prevention and clearance of infection. It appears as though many peptides initially isolated as and termed “antimicrobial peptides” have been shown to have more significant alternative functions in vivo (e.g. hepcidin ).
These peptides are excellent candidates for development as novel therapeutic agents and complements to conventional antibiotic therapy because in contrast to conventional antibiotics they do not appear to induce antibiotic resistance while they generally have a broad range of activity, are bacteriocidal as opposed to bacteriostatic and require a short contact time to induce killing. A number of naturally occurring peptides and their derivatives have been developed as novel anti-infective therapies for conditions as diverse as oral mucositis, lung infections associated with cystic fibrosis (CF) and topical skin infections.
Notes and references
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Antimicrobial_peptides". A list of authors is available in Wikipedia.