"Effects of Bacillus Velezensis D-18 on Health Status of European Seabass (Dicentrarchus labrax) Experimentally Challenged with Vibrio harveyi".

In recent years, the use of probiotics as a possible alternative to antibiotics has generated a growing interest in the global aquaculture field. In this study, the probiotic Bacillus velezensis D-18 was evaluated for its potential protective effect against the marine pathogen Vibrio harveyi. The probiotic was administered through the diet of European seabass (Dicentrarchus labrax) for 30 days, followed by an in vivo challenge with V. harveyi to assess whether the D-18 strain could enhance host resistance to infection. Biofilm formation in tanks was also investigated to analyze its composition and if there are antagonistic interactions between the two bacterial species. From a histological perspective, significant changes were observed in intestinal morphological parameters after infection, the area and base of the villi appeared to increase in the probiotic-fed groups as did the number of goblet cells and in the serum antibacterial activity which was increased in the infected group that received the probiotic compared to baseline levels. The intestinal microbiome was also analyzed to monitor the composition and determine whether different diets before and after infection induced any changes. Although no significant differences were found in the metagenomics of the tank biofilm and the gut microbiome, mortality rates showed that the probiotic provided effective protection against the pathogen. These findings support the potential of B. velezensis D-18 as a viable alternative to antibiotics, particularly when included in the diet prior to disease onset.