An international study led by researchers from the ECOAQUA Institute at the University of Las Palmas de Gran Canaria, in Spain, and the University of Bologna, in Italy, has demonstrated that dietary supplementation with the probiotic Bacillus velezensis D-18 significantly improves the resistance of Europea seabass (Dicentrarchus labrax) against Vibrio harveyi, one of the most concerning emerging pathogens in Mediterranean aquaculture.
In recent years, Vibrio harveyi has established itself as the agent responsible for vibriosis outbreaks that are particularly difficult to control in European seabass and gilthead seabream farming. Unlike other better-known vibrios, this pathogen exhibits a marked ability to form biofilms and persist in rearing systems even in the absence of clinically sick fish. This environmental persistence, combined with a lack of widely effective commercial vaccines and growing restrictions on antibiotic use, has made its control a major health challenge for the sector.
The study, published in the journal Probiotics and Antimicrobial Proteins, evaluated the preventive effect of B. velezensis D-18 in a Recirculating Aquaculture System (RAS).
Fish were fed a diet supplemented with the probiotic for 30 days and subsequently subjected to an experimental challenge with V. harveyi. The results show an increase in survival from 27% in the control group to 50% in the fish that received the probiotic, a statistically significant difference.
Beyond survival data, the study provides physiological evidence that reinforces the consistency of the observed effect. Supplemented fish showed improvements in intestinal morphology, with greater villi development and more favourable goblet cell dynamics, as well as an increase in serum antibacterial activity following infection. These changes suggest the innate immune system was better primed to face pathogen entry.
One of the most significant aspects of the work is that supplementation with B. velezensis D-18 caused no significant alterations to either the fish gut microbiome or the bacterial composition of the environmental biofilm in the tanks. These results is particularly interesting from a production perspective, indicating that the probiotic can enhance host resistance without destabilising the rearing system’s microbial ecosystem, which in the case of V. harveyi also acts as a persistent reservoir for the pathogen.
The authors emphasise that the probiotic’s effect is essentially preventative. Supplementation was carried out prior to the infection challenge, and the study does not evaluate the probiotic as a therapeutic tool nor as a replacement for other health management measures.
Furthermore, production parameters such as growth, feed conversion ratio, or zootechnical performance were not analysed; therefore, the results should be interpreted as an improvement in disease resistance rather than a comprehensive solution to vibriosis.
Overall, the work reinforces interest in probiotics as a complementary tool within health management strategies aimed at reducing reliance on antibiotics in aquaculture. At the same time, it highlights that controlling persistent pathogen like V. harveyi relies more on preparing the fish to withstand infection than on attempting to completely eliminate the bacteria from the system.
Finally, the authors point to the need for larger-scale studies under commercial conditions to confirm the scope of these results and their real-world applicability on farms.
Reference: Bignami, G., Monzón-Atienza, L., Leuzzi, D., Scicchitano, D., Candela, M., Gómez-Mercader, A., Jlidi, M., Gustinelli, A., Tedesco, P., Fioravanti, M. L., Castro, P. L., & Acosta, F. (2025). Effects of Bacillus velezensis D-18 on Health Status of European Seabass (Dicentrarchus labrax) Experimentally Challenged with Vibrio harveyi. Probiotics and Antimicrobial Proteins. Springer Nature. https://doi.org/10.1007/s12602-025-10833-7
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