Probiotics, which are micro-organisms or their products with health benefit to the host, have found use in aquaculture as a means of disease control, supplementing or even in some cases replacing the use of antimicrobial compounds. A wide range of microalgae (Tetraselmis), yeasts (Debaryomyces, Phaffia and Saccharomyces) and Gram-positive (Bacillus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Micrococcus, Streptococcus and Weissella) and Gram-negative bacteria (Aeromonas, Alteromonas, Photorhodobacterium, Pseudomonas and Vibrio) has been evaluated. However, the mode of action of the probiotics is rarely investigated, but possibilities include competitive exclusion, i.e. the probiotics actively inhibit the colonization of potential pathogens in the digestive tract by antibiosis or by competition for nutrients and/or space, alteration of microbial metabolism, and/or by the stimulation of host immunity. Probiotics may stimulate appetite and improve nutrition by the production of vitamins, detoxification of compounds in the diet, and by the breakdown of indigestible components. There is accumulating evidence that probiotics are effective at inhibiting a wide range of fish pathogens, but the reasons for the inhibitions are often unstated

Vibrio harveyi, which now includes Vibrio carchariae as a junior synonym, is a serious pathogen of marine fish and invertebrates, particularly penaeid shrimp. In fish, the diseases include vasculitis, gastro-enteritis and eye lesions. With shrimp, the pathogen is associated with luminous vibriosis and Bolitas negricans. Yet, the pathogenicity mechanisms are imprecisely understood, with likely mechanisms involving the ability to attach and form biofilms, quorum sensing, various extracellular products including proteases and haemolysins, lipopolysaccharide, and interaction with bacteriophage and bacteriocin-like substances

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Members of the ribonuclease III (RNase III) family are double-stranded RNA (dsRNA) specific endoribonucleases characterized by a signature motif in their active centers and a two-base 3' overhang in their products. While Dicer, which produces small interfering RNAs, is currently the focus of intense interest, the structurally simpler bacterial RNase III serves as a paradigm for the entire family. Here, we present the crystal structure of an RNase III-product complex, the first catalytic complex observed for the family. A 7 residue linker within the protein facilitates induced fit in protein-RNA recognition. A pattern of protein-RNA interactions, defined by four RNA binding motifs in RNase III and three protein-interacting boxes in dsRNA, is responsible for substrate specificity, while conserved amino acid residues and divalent cations are responsible for scissile-bond cleavage. The structure reveals a wealth of information about the mechanism of RNA hydrolysis that can be extrapolated to other RNase III family members.