Comparative evaluation of the immune responses and disease resistance of Nile tilapia (Oreochromis niloticus) induced by yeast β-glucan and crude glucan derived from mycelium in the spent mushroom substrate of Schizophyllum commune

Abstract

β-glucan has been applied as an immunostimulant that enhances the fish immunity to contend with bacterial diseases, which have caused devastating losses in diverse economic cultured fish worldwide. The Schizophyllum commune or split gill mushroom is widely cultured in southern Thailand. A large number of undesirable waste, including spent mushroom substrate (SMS), increases along with its production. This work was designed to investigate the immunostimulatory effect of glucan extracted from S. commune cultivation waste in Nile tilapia (Oreochromis niloticus) to discover an alternative source of low-cost β-glucan. Crude glucan, Cr-glucan, which is obtained through pressurized hot water extraction, comprises mainly β-glucan and residues of α-glucan, protein and phenolic compounds. The immunostimulatory potential of Cr-glucan was determined compared with that of commercial β-glucan from Saccharomyces cerevisiae (Yb-glucan) by intraperitoneally (i.p.) injection (10 μg per fish). Immune parameters in blood and skin mucus and the transcriptional expression level of immune-related genes were examined at 6, 12 and 24 h post injection (hpi). The results showed that Cr-glucan could induce these immune parameters similar to Yb-glucan which was significantly higher (P < 0.05) than those of the control fish. In addition, both glucans upregulated all studied genes. Cr-glucan could extend the expression of immune-related genes over those induced by Yb-glucan. The disease resistance against a pathogenic bacterium Aeromonas veronii, a disease-causing agent of motile Aeromonas septicemia (MAS), was also established. The survival proportion in Cr-glucan treatment was higher than that of Yb-glucan and control PBS. The results together suggested that Cr-glucan enhanced the immunity contributing to protect Nile tilapia against A. veronii. These findings provide the first evidence to promote the feasibility of crude glucan from valueless waste as a potent immunostimulant that might be used in tilapia culture.