The in vivo and in vitro pathogenic activities of whole cells and extracellular products of Vibrio alginolyticus for cultured gilt-head sea bream were evaluated. The 50% lethal doses ranged from 5.4 × 104 to 1.0 × 106 CFU/g of body weight. The strains examined had the ability to adhere to skin, gill, and intestinal mucus of sea bream and to cultured cells of a chinook salmon embryo cell line. In addition, the in vitro ability ofV. alginolyticus to adhere to mucus and skin cells of sea bream was demonstrated by scanning electron microscopy. The biological activities of extracellular products of V. alginolyticus were hydrolytic activities; the products were able to degrade sea bream mucus. V. alginolyticus was cytotoxic for fish cell lines and lethal for sea bream. Moreover, the extracellular products could degrade sea bream tissues. However, experiments performed with the bath immersion inoculation technique demonstrated that V. alginolyticus should be considered a pathogen for sea bream only when the mucus layer is removed and the skin is damaged.

Bacteria isolated from an outbreak with moderate mortalities in farmed sole, Solea senegalensis (Kaup), in the south of Spain were identified as Vibrio harveyi and V. parahaemolyticus. Only bacterial strains showing swarming were virulent in sole and caused mortalities in experimentally inoculated fish. However, the signs of the disease were only reproduced with V. harveyi. The intramuscular inoculation of the extracellular products (ECPs) of both species produced mortalities in inoculated fish and the appearance of surface ulcers in the case of V. harveyi. However, the inoculation of sublethal doses of ECPs to fish showed a protective effect against V. harveyi.

Pleuronectiforms are an important group of fish, and one of their species, Solea senegalensis (Kaup 1858), has been extensively studied at different levels, although information about its intestinal microbiota and the effects of different factors on it is very scarce. Modern aquaculture industry demands strategies which help to maintain a microbiologically healthy environment and an environmentally friendly aquaculture. In this context, probiotics seem to offer an attractive alternative. The intake of probiotics could modify the composition of the intestinal microbiota, which is a key component in excluding potential invaders and maintaining health. The aim of this study was to evaluate by 16S rRNA gene analysis using polymerase chain reaction-denaturing gradient gel electrophoresis the effect of administering fresh or lyophilized cells of Pdp11 on the intestinal microbiota of farmed Senegalese sole, using sodium alginate to facilitate the incorporation of bacterial cells to the feed. The results obtained showed that the composition of fish intestinal microbiota was affected when fish received a diet supplemented with sodium alginate and fresh or lyophilized probiotic cells. In all cases, the dominant bacterial groups belonged to γ-Proteobacteria and mainly the Vibrio species. The use of sodium alginate reduced the incidence of populations with <97% 16S rRNA gene sequence identity to uncultured microorganisms in the intestinal microbiota until non-detected limits. On the other hand, the supplementation of the diet with probiotics produced an increase of the predominant species related to Vibrio genus.

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