Dose-dependent influences of dietary β-1,3-glucan on innate immunity and disease resistance of hybrid striped bass Morone chrysops x Morone saxatilis

To investigate potential use of dietary beta-1,3-glucan for health management of hybrid striped bass, juvenile fish were fed diets supplemented with yeast glucan (MacroGuard (R)) at 0.05%, 0.1% or 0.2% of diet for 4 weeks, followed by immune response assays and a bath challenge with Streptococcus iniae. Dietary glucan significantly (P < 0.05) enhanced neutrophil oxidative radical production, and fish fed 0.1% glucan had a significant (P < 0.05) reduction in mortality (10%) after bacterial challenge compared with fish fed the control diet (46.7%). However, accumulative mortality of fish fed 0.2% glucan was not significantly different from that of fish fed the control diet. To further elucidate this observation, macrophages from sub-adult hybrid striped bass were isolated and cultured in L-15 medium with 10% foetal calf serum and penicillin/streptomycin supplemented with 0.2, 0.5, 1, 2, 5, 20 and 100 mu g soluble glucan (MacroGuard (R)) mL-1 for 24 and 48 h. Intracellular superoxide anion production was significantly (P < 0.001) increased by 0.5 mu g glucan mL-1, but significantly (P < 0.001) suppressed by doses > 5 mu g glucan mL-1. It is concluded that dietary yeast glucan has potential for use in diet formulations of hybrid striped bass to limit the adverse effects of S. iniae, but dosage should be an important consideration in administration.