Feed enriched by autochthonous probiotic bacteria and its effect on growth and immune response under different feeding regimes and immune response after Aeromonas salmonicida infection in rainbow trout (Oncorhynchus mykiss)

Abstract This study examined the impact of a newly developed probiotic feed using the autochthonous probiotic strain Lactobacillus plantarum R2 Biocenol™ on the immune response of healthy rainbow trout, particularly after infection with Aeromonas salmonicida subsp. salmonicida . A probiotic feed was tested using different application strategies—continuous and cyclic feeding—to assess its influence on immune function. In the first phase, a non-infectious experiment was conducted to compare these feeding approaches and their effects on immune-related gene expression over time. The second phase involved an infection trial in which fish previously supplemented with probiotics were exposed to a bacterial pathogen to evaluate potential protective effects. Results indicated that while continuous probiotic feeding did not significantly alter immune function, cyclic feeding was associated with a temporary boost in immune response, as evidenced by changes in the expression of key immune-related genes ( cd8 , igm , il-8 , tnf-α ). Furthermore, the body length and weight improved in fish fed with probiotics increased over the study period. One week post-infection, pro-inflammatory cytokine expressions ( il-1 , il-8 , tnf-α ) were reduced in the probiotic group, with a similar trend observed for other pro-inflammatory molecules. The second infectious sampling indicated increased expression of cd4 and cd8 genes, suggesting heightened acquired immunity in the probiotic group. These findings suggest that probiotics can influence immune function and may contribute to improved disease resistance in aquaculture.