Bacillus subtilis supplemented feeding as a method to increase IgM titers and affinity in response to fish vaccination

In aquaculture, the use of probiotics in supplemented diets has been shown to be a suitable strategy to increase the immune status of fish and thereby reduce the impact of pathogens. Specifically, the immunostimulatory effects of the probiotic microorganism Bacillus subtilis have been widely confirmed both in vitro and in vivo in many aquacultured species. However, whether feeding fish with probiotic-enriched diets affects the adaptive immune response mounted to a vaccine has been scarcely addressed in fish. Therefore, in this study, we addressed this using rainbow trout (Oncorhynchus mykiss) as a model. To this aim, fish were fed a probiotic-supplemented diet or a control diet for 30 days and thereafter immunized through different administration routes with different antigenic models, including 2,4,6-trinitrophenyl lipopolysaccharide (TNP-LPS), a Yersinia ruckeri bacterin or a DNA vaccine against viral haemorrhagic septicaemia virus (VHSV). The effects of the B. subtilis-supplemented diet on the systemic specific IgM responses mounted were then established. For TNP-LPS, we also determined the effects of the diet on antibody affinity using a BIAcore instrument, which allows direct detection of antibody-antigen interactions by surface plasmon resonance (SPR) changes. The results presented reveal beneficial effects of feeding this probiotic on the vaccine-induced antibody response and point to the usefulness of designing holistic vaccination protocols that not only focus on antigen optimization or administration regimes, but also include diet composition as an important factor to influence the outcome of the immunization strategy.