WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2018 67 resistance has been reported in pathogenic bacteria of fish such as Aeromonas salmonicida, A. hydrophila, Vibrio anguillarum, Pseudomonas fluorescens, Pasteurella piscicida, Edwardsiella tarda (Aoki 1988) and Yersinia ruckeri (DeGrandis and Stevenson 1985). The presence of multiple antibiotic-resistance bacteria in aquaculture food products has become a serious human health threat because there is a potential that genes responsible for drug resistance may be transferred to bacteria in humans (Kathleen et al. 2016). Although the number of multi-drug resistant bacteria is increasing, the invention of new antibiotics has been very low. From 1960s to early 2010, only four new classes of antibiotics were introduced and none have made any substantial impact (Fig. 1). The global antibiotic market is still largely dominated by antibiotics discovered before the 1960s and the few antibiotics produced since then were synthetic derivatives of existing core structures (Fischbach and Walsh 2009). In this regard, developing and finding new compounds with broad-spectrum antimicrobial activity is a pressing need. This scenario even worse in the case of aquaculture because only one antibiotic (oxytetracycline hydrochloride) is approved by the US Food and Drug Administration for use against different bacterial diseases. Hence, it is the utmost priority to find an alternative antimicrobial agent to reduce production losses from disease epizootics in aquaculture systems. Antimicrobial Peptides as Novel Antimicrobial Agent Antimicrobial peptides (AMPs) are a group of highly conserved oligopeptides with five to over 100 amino acids. They are pivotal humoral components of innate immunity and have been extensively studied in invertebrates and vertebrates including fish and exhibit broad-spectrum antimicrobial activity in vitro and in vivo. They are cationic molecules that can selectively attack negatively-charged bacterial cell membranes and brings about destruction of the organism by pore formation or destabilization of membrane equilibrium or by penetration into the cell and Intensification in aquaculture systems has brought into question the overall well-being of fish, human and the environment. The need to produce more in the culture system imposes unintentional consequences to the fish which weakens the immune system of fish and finally leads to disease outbreak. Furthermore, introduction of non-native fish species also introduces many transboundary aquatic animal diseases. Up to 40 percent of shrimp aquaculture production is lost annually, worth more than $3 billion, mainly due to viral pathogens (Stentiford et al. 2012). Disease losses in aquaculture can account for about 30 percent of total operating costs (Lee and O’Bryen 2003). To efficiently control disease outbreaks in aquaculture systems, a “prevention is better than treatment” approach based on the principle of prophylaxis is the method of choice. As examples, immunostimulants, probiotics, prebiotics and vaccines are being used for disease prevention. However, especially in intensive culture environments, mass mortality occurs even after taking precautions. To prevent the stock being wiped out after a disease outbreak, use of chemotherapeutics such as antibiotics, antiviral drugs and other chemical therapeutants like formaldehyde, sodium chloride, potassium permanganate are used. The gross global investment in aquatic animal health products is $274.4 million for antibiotics and $29.4 million for antiparasitics (The Fish Site 2010). Apart from these drugs, next-generation antimicrobials and antimicrobial peptides with broad-spectrum activity are in focus as alternate therapeutic agents in aquaculture. Antimicrobial Resistance and Need for Novel Antimicrobial Agents in Aquaculture Antimicrobial drugs have been the most common choice for treating many bacteria-borne diseases in people and livestock, including aquaculture, for over 50 years with tremendous benefits. But inappropriate administration of different classes of antimicrobials in aquaculture provides a selective pressure, creating a reservoir of multiple antimicrobial-resistant bacteria in cultured fish and shrimp and in the culture environment. Antimicrobial resistance is rising alarmingly. In aquaculture, antibiotic (CONTINUED ON PAGE 68) Antimicrobial Peptides: A Promising Future Alternative to Antibiotics in Aquaculture Anutosh Paria, Vinay T.N., Sanjay K. Gupta, Tanmoy Gon Choudhury and Biplab Sarkar FIGURE 1. The lack of introduction of potent new antibiotics in the latter half of last century. (Adapted from Fischbach and Walsh 2009.)
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