WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2016 57 Silver Nanoparticles as an Antimicrobial Agent Against Fish Pathogens Fish disease is one of the major hurdles to the sustainable development of aquaculture, causing losses of millions of dollars annually. The appearance and increase in antibiotic resistance are a distressing concern in managing fish diseases. Antimicrobial drugs can cause a serious problem in the aquatic environment because of the rapid spread of antibiotics through the water and development of antimicrobial resistance. Metallic silver as an antimicrobial agent has been documented for more than a decade (Lansdown 2002) and a number of silver compounds are now used in daily life such as silver dressings, silver nitrate, silver zeolite and silver nanoparticles, for a diversity of antimicrobial purposes (Kim et al. 2007, Rai et al. 2009). Due to the increase in the occurrence of bacterial diseases in commercial aquaculture and the development of bacterial resistance, new antimicrobial agents are required. One option is to use nanoparticles of antimicrobial drugs. Silver nanoparticles are one of the most effective nanoparticles because they have a good antimicrobial efficacy against some bacteria, viruses and other eukaryotic microorganisms (Gong et al. 2007). Silver nanoparticles have antimicrobial activity against E. coli, a typical Gram-negative bacteria (Sondi et al. 2004). Aeromonas hydrophila is a Gram-negative bacterium that can grow in aerobic and anaerobic conditions and cause disease in many cultured fish species. Aeromonas hydrophila is a primary or secondary cause of ulcers, fin rot, tail rot, and hemorrhagic septicaemia in fish. Sarkar et al. (2012) observed the inhibitory role of silver nanoparticles in fish against A. hydrophila. Silver nanoparticles synthesized from tea leaf extracts against pathogenic Vibrio harveyi and its protective efficacy on juvenile of Indian white shrimp Fenneropenaeus indicus was observed (Vaseeharan et al. 2010). The inhibitory effect of silver nanoparticles on bacterial fish pathogens like Streptococcus iniae, Lactococcus garvieae, Yersinia Ruckeri and A. hydrophila have been already evaluated (Soltani et al. 2009). Synthesis of Silver Nanoparticles The development of persistent procedures for the synthesis of silver nanomaterials is an important aspect of current nanotechnology research. One such promising process is “green” (organic) synthesis of silver nanoparticles. Silver nanoparticles can be synthesized by physical, chemical and biological methods (Fig. 1). However, for the past few years, various rapid chemical methods have been replaced by green synthesis to avoid toxicity of the process and increase quality. Synthesis of silver nanoparticles using various plants and their extracts can be advantageous over other biological synthesis processes that involve the very complex procedures of maintaining microbial cultures. Silver nanoparticles have been synthesized from naturally occurring sources and their products like green tea Camellia sinensis, neem Azadirachta indica, leguminous shrub Sesbania drummondii, various leaf broths, natural rubber, starch, Aloe vera plant extract, lemongrass leaves extract, inter alia (Vijayaraghavan et al. 2012, Rather et al. 2016). Antony et al. (2013) studied antimicrobial activity of Leucas asperaengineered silver nanoparticles against A. hydrophila infections in catla Catla catla. In-vivo analysis of biochemical parameters and histological architecture provided evidence for the antibacterial effect of silver nanoparticles in catla. Neem-constructed silver nanoparticles (Fig. 2) have immunomodulatory and antibacterial activity in fish (Rather et al. 2016). Concerns/Issues About the Use of Silver Nanoparticles Several inorganic and organic nanoparticles such as silver, gold, copper, iron chitosan, PLGA and titanium dioxide have been used for different applications in aquaculture (Rather et al. 2013, Remay et al. 2013, Rather et al. 2014, Bhat et al. 2016, Rather et al. 2016). Increasing applications of these nanoparticles have led to their accumulation in water and toxicity to aquatic animals. Sharma et al. (2016) revealed that the toxicity of silver nanoparticles has organismsize and dose-dependent relationships. To mitigate the toxicity of silver nanoparticles, biological synthesis is the better option (Fig. 2). Despite these potential pitfalls, the antimicrobial properties of silver nanoparticles against major fish pathogens generates new hope for its possible application in fish health management. (CONTINUED ON PAGE 58) FIGURE 1. Different methods for synthesis of nanoparticles. FIGURE 2. Green synthesis of different silver nanoparticles.
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