GAS1 : a new beta-glucan immunostimulant candidate to increase rainbow trout (Oncorhynchus mykiss) resistance to bacterial infections with Aeromonas salmonicida achromogenes

beta-glucans are prebiotic and/or food additives used by the aquaculture industry to enhance the immune response of fish. Their efficiency may vary according to their origin and structure. In this study, the immunostimulant effects of two beta-glucan types extracted from wild-type baker's yeast (Saccharomyces cerevisiae) and its null-mutant Gas1 were investigated. Gas1 has a beta-1,3-glucanosyltransferase activity necessary for cell wall assembly. Using a positive (commercial product MacroGard(R)) and a negative control (a diet without glucans), we evaluated the immune responses and disease resistance of rainbow trout juveniles (mean weight, similar to 44 g) fed control, low (0.2%) and high (0.5%) doses of Macrogard(R), Gas1, and Wild type-beta-glucan after a short-term (15 days, D15) or mid-term (36 days, D36) feeding periods. We found that beta-glucan supplemented diets did not affect growth performance, mortality, splenic index, or leukocyte respiratory burst activity on D15 nor D36. However, each beta-glucan triggered different immune effectors, depending of the doses or length of exposure compared to others and/or the negative control. Indeed, high dose of MacroGard(R) significantly increased lysozyme activities at D15 compared with the control and other diets (p<0.05). At D36, MacroGard beta-glucan enhanced the production of lymphocytes in comparison with the control diet (p<0.05). Regarding WT beta-glucan, at D36, WT-beta-glucan, especially the high dose, provided the highest enzymatic activities (lysozyme and ACH50) and Ig level (p<0.01). Furthermore, on D36, Gas1 also increased lysozyme activity, Ig proportion, and some immune genes (mcsfra, hepcidin) compared with MacroGard(R) (p<0.05). Besides, both doses of Gas1-beta-glucans increased the resistance of juveniles to bacterial infection highlighted by a higher survival rate at 14 days post-challenge compared with the control and other types and doses of beta-glucans (p