IMMUNOMODULATION IN RAINBOW TROUT Oncorhynchus mykiss FED BIOPROCESSED SOY DIETS AND CHALLENGED WITH Flavobacterium psychrophilum  

Plant-based feedstuffs used as fish meal (FM) protein replacements have the potential for reducing feed costs and providing comparable performance in aquaculture stocks. Antigenic components may be present in the bioprocessed ingredients following these microbial modifications. Thus, there may be a potential for immunostimulatory enhancements and the ability to enhance the immune response following exposure to aquaculture pathogens.   

A 79-day feeding trial incorporating bioprocessed soybean meal (BP-SBM) diets supplemented with a microbial was performed with juvenile rainbow trout (9.6±0.6g).  The study was conducted in a biosecure recirculating aquaculture system and trout were fed two BP-SBM diets supplemented with exopolysaccharide (EXOP) or β-glucans (βG) (Table 1). On day 69, trout (39.8±0.8g) were challenged with 100μl of inoculum (72h in TYES; OD₅₂₅=0.400) via intraperitoneal injection with F. psychrophilum isolate. Treatment fish were subsampled on day 0, 46, 60, and 79 to collect sera, head kidney macrophages, and organs.  Pronephros-derived macrophages were cultured and assessed for respiratory burst activity (RBA), and tissues and organs were harvested to assess cytokine levels organ histology, respectively.  Serum samples were assessed for alternative complement activity and lysozyme content. Bacterial plate cultures were re-isolated from challenged fish or mortalities to confirm F. psychrophilum infection using MALDI-TOF.

At day 46, the RBA of the supplemented BP-SBM1 treatments performed statistically better than the FM control (P=0.009 and P=0.002). The BP-SBM1 base diet (OD₆₂₀=0.407) was also found to elicit a greater RBA (OD₆₂₀=0.373) than the control diet, although not significantly different. The supplemented BP-SBM1 diets also generated a greater RBA than the supplemented BP-SBM2 diets (P=0.004 for exopolysaccharide and P=0.02 for the β-glucans). Survival was highest in the β-glucans treatment (60.79%) while the BP-SBM and control diet demonstrated the lowest survival  (41.07%). Findings indicate varied innate immune responses as a result of feeding supplemented bioprocessed soybean meals. Histological and cytokine profiles will be presented to demonstrate immunological responses to the F. psychrophilum challenge.