Probiotics in aquaculture: recent progress and outlook

Over the various strategies to modulate the composition of the gut microbiota for better,digestion,immunity,and disease resistance of the host have been investigated in various livestock as well as in human beings.The manipulation of the gut microbiota through dietary supplementation of beneficial microbes is a novel approach not only from nutritional point of view but also as an alternate viable therapeutic modality to overcome the adverse effects of antibiotics and drugs.Probiotics are usually live microorganisms which can confer health benefits on host if administered in adequate amounts.Probiotics have been widely used and accepted in aquaculture.Nowadays,a number of preparations of probiotics are commercially available and have been introduced to fish,shrimp and molluscan farming as feed additives,or are incorporated in pond water.The utility of probiotics has raised aquatic production significantly and lead to high economic benefits.Several studies on probiotics have been published during the last decade.However,the methodological and ethical limitations of animal studies make it difficult to understand the mechanisms of action of probiotics,and only partial explanations are available.Among the numerous beneficial effects of probiotics,modulation of immune system is one of the most commonly purported benefits.The role of probiotics in modulating the immune system has been extensively investigated and reviewed in humans and animals.Most of the earlier studies in fish dealt with growth promoting and disease preventing ability of probiotics.A lot of immunological studies have been performed in several fishes using different probiotics and their potency to stimulate the teleost immunity both under in vivo and in vitro conditions is noteworthy.Perusal of available literatures indicates that several probiotics can enhance both systemic and local immunity in fish either individually or in combination.Probiotics interact with the immune cells such as mononuclear phagocytic cells and polymorphonuclear leucocytes and NK cells to enhance innate immune responses.Like higher vertebrates,certain probiotics can enhance the number of erythrocytes,granulocytes,macrophages and lymphocytes in different fishes.The effects of probiotics in stimulating the systemic immune responses are now well documented in several fish species but that of local gut immunity is rare.Limited attempts due to lack of suitable tools are made to analyze the gut immune responses following probiotics treatment.Studies conducted in recent years indicated that probiotics could stimulate the piscine gut immune system with marked increase in the number of Ig+ cells and acidophilic granulocytes.Probiotics supplementation at early developmental stages can be helpful in increasing specific AGs subpopulations.Probiotic therapy offers a suitable alternative for controlling pathogens and overcoming the adverse consequences of antibiotics and chemotherapeutic agents.In fish culture,probiotics either in diet or bioencapsulation help in achieving natural resistance and high survivability of larvae and post larvae of fishes.Furthermore,the effectiveness of probiotics in terms of protection against infectious pathogens is often attributed to the elevated immunity.Protection against edwadsiellosis,enteric red mounth disease,furunculosis,lactococossi,streptococcosis,and several other diseases are successfully accomplished through probiotics feeding.Probiotics treatment leads to better protection of fish from multiple diseases.The beneficial effects of dietary supplements like probiotics,prebiotics and synbiotics have been recorded in a wide range of animal models,including fish.The future application of probiotics in aquaculture is bright.There is an ever-increasing demand for aquaculture products and a similar increase in the search for aquaculture products and alternatives to antibiotics.The field of probiotics intended for aquacultured animal is now attracting considerable attention and a number of commercial products are available,particularly those directed at shrimp larval culture.Probiotic strains that are already adapted,to the dynamics of an aquaculture production system by natural processes will probably lessen farm management and environmental manipulation practices required to achieve the desired probiotic effect in the final product.Introducing such specifically intended probiotics is bound to favour an increase in the application of probiotics,particularly in mollusk production.According to the claims of producers,these products are safe and effective in supporting the health of aquatic animals.On the other hand,there are doubts on the general concept of probiotics and these claims.Thus,there is clearly a need to increase our knowledge of intestinal microbiology and the effective preparation and safety evaluation of probiotics.This paper mainly reviewed mechanisms,beneficial effects on fry,selection,application,safety of probiotics,status of probiotic studies and applications in marine fish culture,hoping to provide a comprehensive summary of knowledge and application of probiotics,which may be useful for exploitation of probiotics in aquaculture.