Cationic antimicrobial peptides as novel alternatives to antibiotics
      Diseases caused by bacteria were the most prevalent and severe diseases in modern animal agriculture. To prevent and control animal bacterial diseases, antibiotics have been used in feeds as a core component of feed additives since 1950`s, but their uses as routine feed additives have been banned in many countries up till noweg. European Union from 2006, Japan from 2008 and Republic of Korea from 2011, because of public concern over possible antibiotic residual effects and the development of drug-resistant bacteria. This clearly highlights the need for new antibacterial agents with fundamentally different modes of action than those of traditional antibiotics. The enormous demand has triggered worldwide efforts in developing novel antibacterial alternatives (Parisien et al. 2008).

Probiotics, prebiotics, acidifying agent, combined enzymes, high concentration of copper and zinc, bacterin, yolk antibody and some Chinese herbal medicines have been introduced as alternatives to antibiotics; however, their effects on animal production are not consistent, resulting in uncertainties and scepticism for development of the products (Jin et al., 1997). Cationic antimicrobial peptides (CAMP) are a promising family of natural antimicrobials as alternatives to antibioticsParisien et al., 2008.

Compared with the currently used antibiotics, CAMP have the advantages as follows:

() The number of CAMP found has been up to 2253 (200 from bacteria295 from plants and 1699 from animals)http://aps.unmc.edu/AP/main.php , accessed August, 2013. They are of vast diversity in term of size, structure, mode of action and specificity, so are rich sources for development of alternatives to antibiotics.

() They are safer to both animals and consumers, because they do not enter the systemic circulation after feeding, without residues in animal productsWakabayashi et al., 2004); they are low in immunogenicity and toxicity to eukaryotic cells (Vaucher et al., 2011).

 () The membrane-active mechanism of action Fig. 1 make it rather difficult for bacteria to become resistant to them (Hancock et al., 2000).

() They could neutralize endotoxins (lipopolysaccharide from Gram-negative bacteria, and lipoteichoic acid and peptidoglycan from Gram-positive bacteria) and inhibit the production of cytokines such as TNF-a and IL-6 by macrophages, subsequently inhibit septic shock during bacterial infections, and promote voluntary feed intakeFig. 2(Hancock et al., 2000).

() They could promote growth of intestinal villiWen et al., 2012).

() Some pathogenic bacteria could down-regulate the expression of genes for intestinal CAMP productionMeade et al., 2009), then disrupt the intestinal barrier functionFig. 3 and Fig. 4(Koslowski et al., 2010) and cause infection, but sufficient supplementation of CAMP could effectively eliminate the pathogens and promote the heath and growth of animals.

Fig. 1. The membrane-active mechanism of action of cationic antimicrobial peptides.

Fig. 2.  Model outlining the major events in induction of sepsis by bacteria and the points at which cationic peptides are proposed to intervene (Hancock et al., 2000).

Fig. 3. Schematic diagram of a small intestinal crypt and its intestinal barrier function (Koslowski et al., 2010).

 

Fig. 4. Schematic diagram of intestinal barrier function

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