WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2017 21 (CONTINUED ON PAGE 22) Nutrient and Enzymatic Contributions to Digestion It is well established that probiotics are beneficial to the intestinal health of host animals. Among the beneficial bacteria, Agrobacterium sp., Brevibacterium sp., Clostridium sp., Microbacterium sp., Pseudomonas sp., and Staphylococcus sp. can contribute to the host’s nutrition, especially in supplying fatty acids and vitamins to host cells (Sakata 1990, Ringø et al. 1995). Some fish gut microbiota may participate directly in the digestion processes of fish. Enzymeproducing microbiota such as Bacillus and Enterobacteriacea (Acinetobacter sp., Aeromonas sp., Flavobacterium sp., Photobacterium sp., Pseudomonas sp., Vibrio sp., Microbacterium sp., Micrococcus sp., Staphylococcus sp.), and some unidentified anaerobes and yeasts are potential contributors (Ray et al. 2012). The metabolic and physiological roles of fish gut microbiota have been the subject of several studies. These microbiota are able to stimulate gut epithelial differentiation and proliferation, gut motility, protein uptake, nutrient metabolism, and innate immunity (Rawls et al. 2004, Rawls et al. 2006, Bates et al. 2006). However, these functional roles are mostly limited to the initial stage of fish fry. In bivalves and crustaceans, microbiota in the gut facilitate digestion by producing extracellular enzymes such as proteases and lipases, as well as providing necessary growth factors (Prieur et al. 1990, Wang et al. 2000). Despite numerous studies demonstrating that microbial activity in the digestive tract may be an important source of nutrients and enzymes to the host, it is difficult to attribute the exact contribution of gastrointestinal microbiota in exothermic animals because of the complexity and variable ecologies of the digestive tracts of different fish species (Ray et al. 2012). Immune Response Enhancement Numerous studies using in vitro models and rats indicate that probiotics have potential benefits to the non-specific immune system. As in many animal studies, the intimate association of probiotics with the intestinal mucosa, alongside intestinal cellular components, appears to improve immunological function in the GIT (Klaenhammer 2007). The immune response is largely specialized to gut-associated lymphoid tissues (GALT) (Famularo et al. 1997). Certain infections are protected by serum, an effect called humoral immunity. The humoral immunity reaction is mediated by circulating antibodies known as immunoglobulins (Ig). Five immunoglobulins (IgA, IgD, IgE, IgG and IgM) play different roles in the humoral immune response system (Devereux 2002). Similar immune responses also have been observed in aquatic animals, when the disease protection mechanism is activated by both cellular and humoral immune defenses (Rengpipat et al. 2000). The immune response of fish can be up-regulated through supplementation of probiotics, either in the form of monospecies or multispecies mixtures (Nayak 2010). Phagocytic, lysozyme, complement, respiratory burst activity and the expression of various cytokines in fish can be stimulated by different probiotics (Nayak 2010). Phagocytic activity of leucocytes increased after oral administration of Clostridium butyricum bacteria to rainbow trout, which subsequently enhanced the resistance of fish to vibriosis (Sakai et al. 1995). Similarly, administration of a mixture of bacterial strains (Bacillus and Vibrio sp.) positively influenced the protective effect of white shrimp against the pathogens V. harveyi and white spot syndrome virus (WSSV) (Balcázar 2003). This protection is largely attributed to the stimulation of the immune system by phagocytosis and antibacterial activity. Administration of a lactic acid bacterium Lactobacillus rhamnosus (strain ATCC 53103) stimulates respiratory burst activity in rainbow trout Oncorhynchus mykiss (Nikoskelainen et al. 2003). Some bacteriocin-like inhibitory substances (BLIS) such as antimicrobial peptides, proteins, or protein complexes excreted/ synthesized by probiotics are also effective to control several fish diesease, including V. parahaemolyticus (Carraturo et al. 2006), Flavobacterium sp. (Balakrishna and Keerthi, 2012) and Aeromonas hydrophila (Selvendran and Michael Babu 2013). Antiviral Effects Viral infection is one of the most common diseases in aquatic environments, seriously affecting aquaculture worldwide (Maeda 2001). Major viral diseases affecting aquaculture include White Spot Syndrome Virus (WSSV), Infectious Pancreatic Necrosis Virus (IPNV), Novirhabdovirus (VHSV), Rhabdovirus (IHNV), Nodavirus FIGURE X. Dietary supplementation with probiotic could improve overall growth performance, survival rate and disease resistant of farmed fish. FIGURE 4. Application of probiotics helps to improve shrimp production yield. Photo by Loh J.Y.
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