The term “probiotic” was first introduced by Parker in 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”. 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.

Probiotics can bind to colonic cell lines and to mucin, which is thought to aid colonization of the animal gut system. The microflora of the gastrointestinal tract (GIT) of aquatic animals can be modified by ingestion of other microorganisms. However, the dominant bacteria in fish / shrimp intestine are quite different from those observed in mammals. 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  Competitive exclusion by potential probiotic bacteria can be evaluated by in vitro antagonistic screening against multiple strains of pathogenic bacteria.

Bacterial antagonism is a common phenomenon in nature. Probiotics may prevent opportunistic pathogens from colonizing the surface of eggs or the GIT by producing antimicrobial compounds or by outcompeting them for nutrients or mucosal space . Certain pathogens produce proteolytic enzymes that can dissolve and digest the bacteria that approaches them, the most remarkable being Streptococcus sp and Bacillus pyocyaneus.

Lactic acid bacteria, notably Lactobacillus sp., Bifidobacterium sp. and Streptococcus sp. are effective against diseases caused by Vibrio sp. (Gatesoupe,1994; Olsson et al., 1998). Bacillus subtilis BT23 has inhibitory effects against vibriosis in tiger shrimp Penaeus monodon (Vaseeharan and Ramasamy 2003).

Nutrient and Enzymatic Contributions to Digestion

Some fish gut microbiota may participate directly in the digestion processes of fish. Enzyme-producing microbiota such as Bacillus some unidentified anaerobes and yeasts are potential contributors.      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.

Immune Response Enhancement

The immune response of fish can be up-regulated through supplementation of probiotics, either in the form of monospecies or multispecies mixtures. Phagocytic, lysozyme, complement, respiratory burst activity and the expression of various cytokines in fish can be stimulated by different probiotics. Phagocytic activity of leucocytes increased after oral administration of Clostridium butyricum bacteria to rainbow trout, which subsequently enhanced the resistance of fish to vibriosis. 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). Administration of a lactic acid bacterium Lactobacillus rhamnosus (strain ATCC 53103) stimulates respiratory burst activity in rainbow trout Oncorhynchus mykiss. 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, Flavobacterium sp. and Aeromonas hydrophila.

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