Effects of Dietary Yeast Culture on Humoral and Mucosal Immunity, Gut Microbiota, Hepatic Antioxidant and Stress Responses in Rainbow Trout, Oncorhynchus mykiss

The yeast Saccharomyces cerevisiae is known as a probiotic for growth promotion, gut health, and immunomodulation. There are various strains of yeast with variable properties, which can be examined in aquaculture. So, the present study aimed to assess, for the first time, the effects of S. cerevisiae (PTCC 5052) as a dietary probiotic on immunological, antioxidant and stress responses of rainbow trout, Oncorhynchus mykiss. The fish were fed diets supplemented with 10⁷ (Y7) and 10⁸ (Y8) colony-forming units/g yeast for 30 days. Then the fish were subjected to a crowding stress for 24 h. The results showed that dietary yeast supplementation had no significant effects on the fish growth performance, blood total and differential leukocyte count, and plasma/mucus alternative complement activity (P>0.050). However, dietary yeast significantly decreased gut total viable bacteria and Vibrio sp., and significantly enhanced plasma/mucus lysozyme, bactericidal activities and gut lactic acid bacteria count (P<0.050). Dietary yeast also significantly improved hepatic antioxidant parameters and mitigated hepatic lipid peroxidation (P<0.050). After the stress, Y7 and Y8 treatments had significantly lower stress responses, including plasma cortisol, glucose, alanine aminotransferase and aspartate aminotransferase, compared to the control group (P<0.050). In summary, this research demonstrated that dietary yeast can modulate gut microbiota and augment immunological, antioxidant and stress tolerance capacities in rainbow trout.