WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2017 19 The Application of Probiotics in Aquaculture The rise in bacterial antibiotic resistance and antibiotic residues in cultured aquatic animals due to extensive use of chemotherapeutic agents has become a global concern. Vaccination and immunostimulant treatment are ideal methods for preventing infectious diseases but their use remains very limited and rather uncommon in aquaculture, especially in Southeast Asia. In recent years, attention has focused on the use of probiotics in the development of strategies for microbial control and to reduce the use of therapeutic chemicals and antibiotics, towards a more environmentally friendly and sustainable aquaculture (Pintado et al. 2010). Many aquaculture farms in Southeast Asia apply probiotics as a prophylactic measure to improve farm production yields (Fig. 1). The term “probiotic” was first introduced by Parker (1974) and the original definition was “organisms and substances which contribute to intestinal microbial balance.” The definition of probiotic was redefined as “live microorganisms which, when administrated in adequate amounts, confer a health benefit on the host” (FAO/ WHO 2001). Most probiotics are supplied as live supplements in feed to benefit the host by inhibiting pathogenic microbes, improving immune response, improving survival and growth rates, enhancing digestion and increasing feed utilization, promoting antimutagenic and anticarcinogenic activity, and improving water quality in culture systems (Harikrishnan et al. 2010, Andani et al. 2012). The mechanisms of action of probiotics are well defined: creation of a hostile environment for pathogens by production of inhibitory compounds (bacteriocins, lysozymes, proteases and hydrogen peroxide), competition for adhesion sites and essential nutrients and further improvement of immune responses of hosts, supplementation of essential nutrients, vitamins and enzymes, direct uptake of dissolved organic materials mediated by beneficial bacteria, and modulating interaction with the environment (Gatesoupe 1999, Gomez-Gil et al. 2000, Irianto and Austin 2002, Balcázar et al. 2006). Most probiotics proposed as biological control agents in The Role of Probiotics and Their Mechanisms of Action: An Aquaculture Perspective Jiun-Yan Loh aquaculture are lactic acid bacteria (Lactobacillus and Carnobacterium) and representatives of the genera Vibrio (V. alginolyticus and V. parahaemolyticus), Bacillus, Pseudomonas, and others including Aeromonas and Flavobacterium (Balcázar et al. 2006, Das et al. 2006, Balcázar et al. 2007). Unlike human probiotics, there are only a few key genera of probiotic bacteria, such as Lactobacillus and Bifidobacterium. Identification of probiotic bacteria requires multiple screening steps to confirm its feasibility for field application. Molecular identification usually follows stringent screening steps to verify the identity of the beneficial bacteria (Figs. 2A and 2B). Mechanisms of Action Competitive Exclusion Probiotics can bind to colonic cell lines and to mucin, which is thought to aid colonization of the animal gut system (Schiffrin et al. 1995, Ouwehand et al. 2000, Juntunen et al. 2001). The microflora of the gastrointestinal tract (GIT) of aquatic animals can be modified by ingestion of other microorganisms. However, the dominant bacteria in fish intestine are quite different from those observed in mammals (Gatesoupe 2005). The composition of microbial communities is greatly influenced by husbandry practices and environmental conditions, e.g. culture water and abiotic or biotic factors, which stimulate the proliferation of selected bacterial species. Therefore, addition of beneficial bacteria to culture water or via feed supplementation during initial egg fertilization or pre-larval stages could have a distinct advantage through the mechanism of competitive exclusion for attachment sites on egg surfaces or in the GIT (Irianto and Austin 2002). Competitive exclusion by potential probiotic bacteria can be evaluated by in vitro antagonistic screening against multiple strains of pathogenic bacteria (Figs. 3A and 3B). Bacterial antagonism is a common phenomenon in nature. Probiotics may prevent opportunistic pathogens from colonizing FIGURE 1. Shrimp aquaculture in Malaysia is heavily dependent on probiotic supplementation as a common practice to improve the shrimp health, production and water quality. Photo: Loh J.Y. (CONTINUED ON PAGE 20)
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