Immunomodulation of rainbow trout (Oncorhynchus mykiss) fry by bath exposure to β-glucan from Euglena gracilis

Early developmental stages of fish mostly depend on innate immune factors for their protection. Augmenting these factors by application of different immunostimulatory substances may be beneficial for rearing and survival of the early life stages of fish. Bath administration of stimulants leads to a uniform exposure of fish independent of feed intake and reduces the individual handling. The present study demonstrates the immunostimulatory effect of beta-glucan (bath exposure) in rainbow trout fry at different dosages and exposure time. Rainbow trout fry (avg. wt. 770mg; 87days post hatch) were exposed to three different concentrations of beta-glucan (10, 100 and 1000 mu g mL(-1)) by bath exposure for 1 and 24h. Expression of immune related genes from pooled internal organ samples of individual fish were analysed using a real time qPCR assay. Expression of complement factors (C3 and factor B) and acute phase proteins (hepcidin, precerebellin and transferrin) was significantly up-regulated after 24h bath stimulation with beta-glucan (100 mu g mL(-1)). These innate immune factors may play a vital role in clearance of pathogens. The expression of most of genes showed both a dose- and time-dependent response. A medium dose (100 mu g mL(-1)) induced a significant increase in expression of complement factors and acute phase proteins mainly at 24h exposure, whereas the highest dose of beta-glucan (1000 mu g mL(-1)) down-regulated the expression of most of the studied genes. The result from the present study indicates that beta-glucan bath exposure could be applied for enhancing the innate immune factors even in fry.