Investigation of the probiotic effects of Lactobacillus sakei 2-3 and Candida zeylanoides Y12-3 strains in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

This study aimed to assess individual and combined effects of candidate probiotic strains C. zeylanoides Y12-3 and L. sakei 2-3 on growth, hematological parameters, serum immunological parameters, serum biochemistry, histopathology, histomorphology, expression of immune and antioxidant enzyme genes, and disease resistance against L. garvieae in rainbow trout. The fish were fed four different feeds (control, L. sakei 2-3, C. zeylanoides Y12-3, and L. sakei 2-3 + C. zeylanoides Y12-3) for 60 days. At the end of the experiment, growth parameters, serum glucose levels, serum lysozyme activity, and expression of immune and antioxidant enzyme genes were significantly increased in the probiotic groups. Additionally, triglyceride levels decreased in the probiotic groups compared to the control group, whereas serum ALT levels did not change. The villus width and the number of goblet cells increased in the proximal intestines of the fish in C. zeylanoides and L. sakei + C. zeylanoides groups. L. sakei 2-3 showed higher superoxide anion production, expression of immune genes (IgM, IL-B1, lysozyme, TNF-alpha, HSP70) in the kidney, antioxidant enzyme genes (GPX, GST, SOD) in the liver compared to C. zeylanoides. TNF-alpha, HSP70), and antioxidant enzyme genes (GPX, GST, SOD) compared to C. zeylanoides. L. sakei and the combination of L. sakei + C. zeylanoides provided resistance to L. garvieae compared to the control group. However, C. zeylanoides was similar to the other two probiotic groups regarding disease resistance against L. garvieae. However, histopathological examinations revealed reversible changes in the proximal intestine, anterior kidney, and liver of fish in the C. zeylanoides and L. sakei + C. zeylanoides groups. Hence, future studies are still required to explore the effects of shorter-term use of the C. zeylanoides strain in rainbow trout to prevent undesirable effects on tissues. In brief, the findings, as mentioned above, showed that L. sakei 2-3 and C. zeylanoides Y12-3 could be potential probiotic candidates for use in rainbow trout farming. Moreover, the probiotic effects of both strains on different fish species should also be studied.