Improvement of the immunological and antioxidant capacity of rainbow trout (Oncorhynchus mykiss) by using the paraprobiotic Bacillus subtilis subsp. spizizenii

The aim of this study was to produce Bacillus subtilis subsp. spizizenii as a paraprobiotic for the nutrition of rainbow trout ( Oncorhynchus mykiss ). Fifty-four trout (74.51 ± 3.15 g) were transferred to nine 300-liter tanks and fed PRO (live freeze-dried form), PAR (irradiated freeze-dried form) and control feed (n=3) for 8 weeks. Growth, hematological, immunological and enzymatic liver functions, as well as antioxidant capacity were then analyzed. Kidney, liver, and intestinal tissue as well as the expression of cytokines (TNF-1, TNF-2, IL-1β1, and IL-1β2) in spleen, and kidney were analyzed. The growth indices, white, and red blood cells, neutrophils, monocytes and granulocytes, total protein and globulin improved in the PAR and PRO groups compared to the control group (p<0.05). The results showed that the PAR and PRO groups resulted in modulation of ALT, AST and ALP compared to the control group (p<0.05). Antioxidant indices were highest in the PAR group compared to the PRO and control groups (p<0.05). Intestinal histology results showed that the height of intestinal folds and goblet cells increased in fish fed PRO and PAR compared to the control group (p<0.05). Although melatonin increased in the PAR and PRO groups, it decreased in the liver of PAR compared to PRO (p<0.05). Immunological indices improved in the PAR and PRO groups compared to the control group (p<0.05), with no differences between PAR and PRO (p<0.05). The results of this study show that the paraprobiotic produced in this study is effective under culture conditions and can therefore be used as a supplement in trout farming. • Paraprobiotic B. subtilis bacteria were produced by gamma irradiation (PAR). • The paraprobiotic performance of PAR was compared with activated live B. subtilis (PRO) under in vitro conditions. • PAR-fed fish showed a better antioxidant and immunological status than PRO.