The screen herbal immunopotentiator and research on its effect on the innate immune system and disease resistance of Nile tilapia (Oreochromis niloticus) against Streptococcus agalactiae

Abstract

To screen potential herbal immunopotentiators, the effects of 20 herbal aqueous extracts (HAEs) on the oxygen respiratory burst activity (ORBA) and phagocytic activity of tilapia blood leucocytes were determined using nitroblue tetrazolium (NBT) reduction and phagocytizing latex beads methods, respectively. The screened-out herbal aqueous extract (HAE) supplemented diet (10 g/kg, dry herb/dry feed) was consecutively fed to healthy tilapia for in vivo evaluating its effects on the ORBA and phagocytic activity of head-kidney leucocytes, the acid phosphatase (ACP), lysozyme (LZM), catalase (CAT) and superoxide dismutase (SOD) activities of serum, and the mRNA expression levels of IL-β, IL-8, Hsp70, and TNF-α in the spleen. For evaluating the efficacy of the screened-out HAE on preventing streptococcosis, the tilapia were respectively and consecutively fed the control diet and the screened-out HAE supplemented diet for seven and fourteen days and subsequently infected with Streptococcus agalactiae. Their survival cures were compared using the log-rank test. Additionally, the components in the screened-out HAE were identified using ultra-performance liquid chromatography-Q exactive orbitrap-mass spectrometry (UPLC-QE-MS). Results showed that, among the candidate HAEs, Scutellaria baicalensis HAE showed more ability in terms of simultaneously enhancing the ORBA and phagocytic activity of tilapia blood leucocytes as compared to the other candidate HAEs. Oral administration of S. baicalensis HAE significantly elevated the ORBA and phagocytic activity of head-kidney leucocytes, the ACP, LZM, and SOD activities of sera, the mRNA expression levels of IL-8 and Hsp 70, and the cumulative survival rate of tilapia with S. agalactiae infections. The negative and positive models of UPLC-QE-MS identified 256 and 596 components of S. baicalensis HAE, respectively. Taken together, the results of this study indicate that S. baicalensis HAE could prove useful in the prevention of tilapia streptococcosis and suggest that it could be further developed as immunostimulants.