68 JUNE 2018 • WORLD AQUACULTURE • WWW.WAS.ORG thereby the impairment of cellular machinery (Reddy et al. 2004) (Fig. 2). Because of the wide activity against different microbial agents, these molecules have made their mark as an emerging class of natural antibiotics. Apart from having an antimicrobial role against many microorganisms, the salient features of AMPs are their broad-spectrum antimicrobial activity towards multi-drug resistant microbial isolates and very minimal chance to allow development resistance due to rapid lysis of cell membranes (Lai and Gallo 2009) (Fig. 2). Antimicrobial peptides (AMPs) have been isolated and characterized as a novel and useful alternative to antibiotics (Mookherjee and Hancock 2007). Even disease-resistant transgenic fish stock was developed to target selected AMPs (Buchanan et al. 2001, Sarmasik et al. 2002). Fish Antimicrobial Peptides Most fish AMPs are potent antibiotics with a broad spectrum of activity at micromole concentrations (Rajanbabu and Chen 2011). Although the first AMP found in fish was purified in 1995 (Pilet et al. 1995), it took almost a decade to learn about its role as a major component of the innate immune response. Piscidins are the family of linear, amphipathic AMPs. The first member of this family was identified from skin mucous secretions of the winter flounder Pleuronectes americanus and called pleurocidin (Cole et al. 1997). Other piscidins homologous to pleurocidins were identified in other teleosts (named asmisgurin, moronecidin, epinecidin and dicentracin). Generally, piscidins are found in gill, skin and intestine of finfish. They have potent antimicrobial activity against a variety of microorganisms. These are specifically effective against Gram-positive and -negative bacteria, with the best antimicrobial activity observed in the case of Streptococcus, Pseudomonas, Bacillus and Vibrio species. Other than bacteria, piscidins have activity against fungi (Niu et al. 2013), parasites (Colorni et al. 2008), and viruses (Wang et al. 2010). Piscidins have immunomodulatory capacity to strengthen the innate immune system by modulating certain immune system-related genes (MassoSilva and Diamond 2014). Hepcidins are cysteine-rich peptides with wide availability among vertebrates including mammals, reptiles, amphibians and fish. The first hepcidin in fish was identified and isolated from hybrid striped bass (Shike et al. 2002) and since then hepcidins have been identified in around 40 other teleosts. In fish two forms of hepcidins can be identified: HAMP1 and HAMP2. HAMP1 is the most abundant member and present in both actinopterygian and non-actinopterygian fish but HAMP2 has been found only in actinopterygian fish. Fish hepcidins are potent antimicrobials against a wide variety of bacterial fish pathogens such as Streptococcus iniae, Yersinia and Pseudomonas, even at low micromole level (Masso-Silva and Diamond 2014). These are also effective against different fungi and viruses. Defensins is the group of cysteine-rich, cationic antimicrobial peptides found in plants, fungi, invertebrates and vertebrates. Although defensins are of three different types (α-, β-, and θ-defensins) in mammals, the defensins in fish belong to the β-defensin group. β-defensins in fish were initially identified from zebrafish, fugu and puffer fish (Zou et al. 2007). Since then many defensins have been identified in many other marine and freshwater fish species. Fish β-defensins are mostly abundant in skin, head kidney and spleen. The antimicrobial activity of this AMP was established against pathogenic bacteria of fish such as A. hydrophila, Y. ruckeri, V. anguillarum and E. tarda and different fish-specific viruses including Singapore grouper iridovirus (SGIV), nodavirus and viral haemorrhagic septicaemia virus (VHSV). Cathelicidins is an unusual group of AMPs with varying structure and size and defined by a conserved domain called ‘cathelin.’ The first member of this group in fish was identified in Atlantic hagfish Myxine glutinosa (Uzzell et al. 2003). The fish cathelicidins characterized by the cathellin domain from different fish species can be subdivided into two groups: the linear peptides and those that exhibit a characteristic disulphide bond (Masso-Silva and Diamond 2014). Fish cathelicidins have antimicrobial activity against fish pathogens such as Y. ruckeri, A. salmonicida, C. albicans and Saprolegnia parasitica. Application as Therapeutics The therapeutic potential of AMPs was evaluated for many critical diseases in mammals and recently these were evaluated for their potential to treat microbial infections in aquaculture. Among piscidins, oral and injection administration or electro-transfer of epinecidin-1 leads to enhanced survival in zebrafish and grouper infected with V. vulnificus and Streptococcus agalactiae (Lin et al. 2009, Pan et al. 2011a, 2012a). Tilapia hepcidin reduces the bacterial count and thereby fish mortality during V. vulnificus infection (Pan et al. 2012b). Transgenic zebrafish bearing tilapia hepcidin have enhanced bacterial resistance to V. vulnificus and S. agalactiae (Pan et al. 2011b). Conclusion Large-scale microbial infection in aquaculture frequently leads to considerable economic losses, as there are very few approved drugs available to counteract such a problem. Harnessing the potential of fish AMPs will give us a novel natural antimicrobial drug to handle this crisis situation against antibiotic-resistant microbes. More pronounced efforts are needed in the future to convert this baseline information into a potent, multifaceted, costeffective drug for aquaculture. FIGURE 2. Biological roles played by different AMPs. (Adapted from Diamond et al. 2009.)
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