PROBIOTIC SELECTION FOR EUROPEAN SEA BASS LARVICULTURE: AN INNOVATIVE APPROACH

Schaeck Marlien*, Van Den Broeck Wim, Haesebrouck Freddy, Decostere Annemie
 
Department of Morphology and Department of Pathology-Bacteriology-Avian Diseases, Faculty of Veterinary Medicine, Ghent University,
Salisburylaan 133, 9820 Merelbeke, Belgium. marlien.schaeck@ugent.be

 

Disease outbreaks are being increasingly recognized as a significant constraint to aquaculture production, affecting the economic development of the sector. Due to the mounting awareness of the risk associated with the use of antibiotics, treatment with probiotics has recently emerged as the preferred environmental-friendly prophylactic approach in disease management and production enhancement. This brought along new questions regarding the mechanisms through which the probiont may act. Indeed, a judicious and scientifically supported probiotic treatment warrants a thorough testing of its efficacy and safety, using standardized and controlled experimental conditions. However, the stochastic colonization of larvae by micro-organisms may entangle the establishment of a reproducible experimental set-up. Therefore, studying the host raised in the absence of bacteria or under gnotobiotic conditions (with a known composition of bacterial microbiota) has become an important tool to unravel the complex host-microbe interactions.

In this respect, we developed a state-of-the-art protocol to generate germ-free sea bass larvae. This model can be successfully applied to expand our knowledge on the underlying microbial signals and responsive host signal transduction mechanisms. As such, our understanding of the host-microbe interactions will be facilitated in the absence of interference by unknown micro-organisms. We isolated and screened 206 bacterial isolates from sea bass larvae for their probiotic potential. Initially, bacterial isolates were selected based on in vitro characteristics including haemolytic and antagonistic activity against several notorious sea bass pathogens. Subsequently, the harmlessness and the protective effect of the probiotic candidate against a pathogen challenge were evaluated in vivo adopting the previously developed gnotobiotic sea bass larval model. A superiority trial was used to detect a difference between the treatments. The untreated group of fish larvae challenged with Vibrio anguillarum presented a low survival of 35.4 %, while the fish treated with the probiotic candidate Vibrio lentus showed a significantly increased survival of 97.9 % following pathogen challenge.

Our results unmistakably revealed the superiority of probiotic treatment with V. lentus, assigning the latter as a probiotic candidate suitable for the preventive treatment of V. anguillarum infections in fish larvae. In this respect V. lentus will be further investigated to unravel its mechanism of action, with a focus on disease resistance in the host organism. This will be realized by applying innovative techniques e.g. immunofluorescence microscopy, microarray analyses and laser capture microdissectioning. The latter is a technique that was recently pinpointed by our research group for the microdissectioning of intestinal tissue of sea bass larvae allowing tissue-specific gene expression analysis. To our knowledge, this is the first study successfully testing probiotic effectiveness in vivo while adopting a gnotobiotic aquatic model.