Short-Term Dietary Inclusion of Methylococcus capsulatus-Derived Single-Cell Protein Modulates Growth and Immune-Gene Expression in Juvenile Rainbow Trout (Oncorhynchus mykiss)

We here report immunomodulatory effects of feeding with microbial single-cell protein (SCP from Methylococcus capsulatus) on female rainbow trout (Oncorhynchus mykiss) following replacement of conventional feed proteins in a 30-day feeding period. Conventional protein ingredients (soy protein concentrate, fishmeal (FM), wheat gluten meal and corn gluten meal) were partially replaced with microbial SCP at graded inclusion levels (0, 4, 6 and 8% SCP). Growth performance parameters, such as final body weight (FBW) and final fish length (FL), were elevated at 6% SCP inclusion. In the present investigation we found that immune gene expression in the liver and spleen was regulated following feeding microbial SCP in a concentration-dependent way. Feed with 4% SCP inclusion induced a significant upregulation of the hepatic expression of IL-10 (p < 0.05), whereas splenic expression of IL-6 and Cathelicidin-2 genes were downregulated. No significant differences were observed at 6% SCP inclusion. At 8% SCP inclusion, splenic expression of multiple immune-related genes, including those encoding Cathelicidin-2, CD8, IgM, IL-6, and SAA, was significantly downregulated, along with hepatic expression of Cathelicidin-1 (p < 0.05). Although immune gene expression was downregulated at 8% SCP inclusion, growth performance was not adversely affected. These findings indicate that up to 8% of conventional feed protein can be replaced with microbial SCP without compromising growth performance, although immune gene regulation varies depending on tissue and inclusion level. Gene regulation and health effects induced by feeding rainbow trout with high microbial SCP inclusion feed and over longer time periods should be further investigated. For this purpose, we recommend studies on functional immune parameters such as vaccine responsiveness and antibody production.