Heat-killed Lactiplantibacillus plantarum as a paraprobiotic feed additive for Atlantic salmon: modulation of host response to a Moritella viscosa challenge

Heat-killed Lactiplantibacillus plantarum strain L-137 (HK L-137) has shown bioactive properties associated with enhanced performance and immunomodulation in aquaculture species. To quantify the impact of this, the current study used vaccinated and unvaccinated pre-smolts Atlantic salmon ( Salmo salar ) fed for 4 weeks in freshwater with a control diet (CD) or diets supplemented with HK L-137 (LP20, 0.02 g kg −1 ) or MacroGard® (MG, 2 g kg −1 ). After seawater transfer, post-smolts were fed solely CD and challenged in a bath with Moritella viscosa , the main bacterial skin pathogen that negatively impacts fish health and welfare, causing significant economic losses in the aquaculture sector. Main results showed no differences in final body weight between the dietary groups (both at the end of the freshwater or seawater phase). Vaccinated fish fed LP20 had a 92% survival rate, whereas CD and MG groups showed 80%. Furthermore, vaccinated fish LP20 or MG showed a delayed progression of bacterial disease compared with vaccinated fish fed the control diet. In addition, a significant interaction among diet, vaccination, and health status was observed for total IgM in serum, with vaccinated diseased fish fed LP20 or MG exhibiting higher levels of this biomarker compared to those fed the control diet. However, the level of specific IgM against M. viscosa did not show any significant main effects for any individual factor or their interactions. Overall, dietary supplementation with HK L-137 as paraprobiotic during the freshwater phase showed potential to modulate the humoral immune response through the production of natural antibodies. This can contribute to enhanced host immune responsiveness and resilience to M. viscosa infection following transfer toseawater, particularly in vaccinated fish.