those that affect humans, are gaining greater acceptance. One of these approaches involves the use of probiotics. Several definitions of probiotics have been proposed. Fuller (1989) gave a precise definition, which continues to be widely used, i.e., ‘a live microbial feed
supplement which beneficially affects the host animal by improving its intestinal balance’. However, Verschuere et al (2000) have expanded this definition to allow for a broader application of the term, as follows: ‘A probiotic is defined as a
live microbial adjunct which has a beneficial effect on the host by modifying the host-associated or ambient microbial community, by ensuring improved use of the feed or enhancing its nutritional value, by enhancing the host response toward disease, or by improving the quality of its ambient environment.’ The means by which probiotics produce positive health benefits in aquaculture have been reviewed by Irianto and Austin (2002). Numerous organisms, including a wide range of microalgae (Tetraselmis spp.), yeasts (Debaryomyces, Phaffia and Saccharomyces spp.), and Grampositive (Bacillus, Carnobacterium, Enterococcus, Lactobacillus, Micrococcus,
Streptococcus and Weissella spp.) and Gram-negative bacteria (Aeromonas, Alteromonas, Photorhodobacterium, Pseudomonas and Vibrio spp.), have been evaluated. The mode of action of the probiotic activities of these agents has not really been investigated, but possibilities include competitive exclusion, ie, the probiotic bacteria actively inhibit the colonization of potential pathogens in the digestive tract by antibiosis or by competition, and/or by the stimulation of immunity in the host. These products may stimulate appetite and improve nutrition by the production of vitamins, by the detoxification of compounds in the diet, and by the breakdown of indigestible components. There is accumulating evidence that probiotics are effective in inhibiting a wide range of bacterial and some viral pathogens in fish (Douillet, 2000; Tae-Kwang Oh [publication date not stated]).
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