Dietary probiotic prototypes and their effects on growth performance, immune function, and gut microbiota of rainbow trout (Oncorhynchus mykiss)

This study focused on developing novel probiotic formulations for rainbow trout aquaculture by testing two formulations containing bacterial strains isolated from the gut of rainbow trout. The effects on growth, immune response, liver enzyme activity, immunity-related gene expression, and gut microbiota composition were evaluated. A completely randomized design was used, involving 450 rainbow trout distributed into 70-l fiberglass tanks with six dietary treatments and three replicates per treatment (total of 18 tanks). Treatments A and B contained three strains of Pediococcus sp. P15, Lactococcus lactis ml3, and Weissella cibaria ml6 administered at two dosages (300 and 500 mg/kg). Treatments C and D included five strains of Pediococcus sp. P15, Lactococcus lactis ml3, Weissella cibaria ml6, Enterococcus faecium E10, and Lactobacillus curvatus 13 A also at 300 and 500 mg/kg. Treatment E was a commercial probiotic at 300 mg/kg, and treatment F served as the control without probiotics. Fish treated with A, B, and E showed significantly higher final weights than the control, with A and B groups exhibiting the lowest feed conversion ratios. Groups A and D had significantly increased immunoglobulin M (IgM) levels. Tumor necrosis factor (TNF) expression decreased in group A, while interleukin 6 (IL6) and immunoglobulin T (IgT) expressions increased by 4.4 and 4.22-fold, respectively. Treatment A also resulted in higher counts of lactic acid bacteria in the gut. Overall, treatment A demonstrated the greatest potential to improve growth, immunity, and gut microbiota, making it a promising probiotic candidate for rainbow trout farming.