To cope with multiple stress responses in fish, aquaculture sector is moving towards increased use of functional aquafeeds. Heat-killed Lactobacillus plantarum strain L-137 (HK L-137), an immunobiotic lactic acid bacterium, has been reported to have immunomodulatory effects in numerous animals, including fish such as Nile tilapia (Oreochromis niloticus) and bighead catfish (Clarias macrocephalus). Dietary supplementation of HK L-137 has also shown to improve growth performance, survival rate, and stress resistance.
The present study was conducted to evaluate the health effect of HK L-137 in salmonids by characterization of immune biomarkers in both in vitro and in vivo experiments. In vitro experiments were performed using RTgutGC, a rainbow trout intestinal epithelial cell line and SHK-1, an Atlantic salmon head kidney macrophage-like cell line. Results showed a positive effect of HK L-137 on gut barrier function as indicated by increased RTgutGC proliferation, transepithelial electrical resistance, and decreased RTgutGC monolayer permeability. HK L-137 also modulated immune responses by up-regulating expression of pro-inflammatory cytokine IL8 and TNFα in SHK-1 cel ls and decreasing the protein production of anti-inflammatory molecule Anxa1 in RTgutGC. A 9-week fish experiment with increasing dietary levels of HK L-137 were conducted (D1 0mg kg-1, D2 4mg kg-1, D3 20mg kg-1, D4 100mg kg-1, D5 2g kg-1 MacroGard®). No differences in growth performance were found among the dietary groups. However, a decrease in Anxa1 protein level in the distal intestine and an increase in non-specific IgM in the plasma were observed (Fig1), indicating an interesting immunostimulant property of HK L-137 in Atlantic salmon. 16S rDNA sequencing of gut microbiota revealed that the dominant microbial phyla are Proteobacteria, Firmicutes, and Actinobacteriota. Regarding alpha- and beta-diversity, we observed a subtle dose-response relationship. D4 had a higher richness, similar to D5, while D2 resembled the control diet.
The results from the present study suggest that HK L-137 is a promising functional additive that can improve health and welfare for Atlantic salmon.