Abstract
In order to explore the mechanism of prevention and treatment of Streptococcus agalactiae disease of Tilapia by water extract of Scutellaria baicalensis, A total of 104 GIFT Tilapia with body weight of about 60 g were randomly divided into 4 groups: blank control group, negative control group, control group and model group, with 26 fish in each group. The negative control group and control group were fed a diet containing 5% Scutellaria baicalensis water extract, and the blank control group and model group were fed a basic diet. The challenge test was conducted on the 8th day after feeding; the blank control group and the negative control group were injected with sterile normal saline peritoneally, and the control group and the model group were injected with 1.5×10~9 cfu/mL of Streptococcus agalactiae bacteriae solution, 0.3 mL/tail, and observed for 10 days. During the experiment, clinical symptoms and death of each group were recorded, and blood was collected from the tail vein of dying test fish. The dead test fish were dissected. After the experiment, the mortality rate of each group and the protection rate of 5% Scutellaria baicalensis water extract to the control group were calculated, and the tail vein blood was collected for other test fish. The blood of each group was analyzed by liquid chromatography mass spectrometry（LC-MS）, and the differential metabolites were screened by constructing principal component analysis（PCA） model and partial least squares discriminant analysis（PLS-DA） model, and the pathway enrichment analysis was performed after the KEGG database was imported. The results showed that the blank control group and the negative control group had good appetite and no abnormal symptoms, and the mortality rate was 0. In the model group, the movement was slow; the appetite was decreased; the body surface was broken and bleeding; the eyeball was protruding; the body was out of balance, and the behavior of side swimming or circle swimming appeared. Necropsy showed edema and bleeding in liver, spleen and intestinal tissues of different degrees, and the mortality rate was 95.45%. The symptoms of the control group were the same as those of the model group. Some fish die; some fish were relieved（the appetite slowly returned to normal, and the mental state was good）. The mortality rate was 46.15%. The protection rate of 5% scutellaria scutellaria water extract to Tilapia was 51.65%. In the positive and negative ion mode, 7 652 ion peaks were detected and 998 metabolites were identified. Among them, there were 348 different metabolites between the model group and the blank control group, 257 different metabolites between the negative control group and the blank control group, 241 different metabolites between the treatment group and the blank control group, 275 different metabolites between the treatment group and the model group, and 190 different metabolites between the treatment group and the negative control group. There were 377 different metabolites between negative control group and model group. Compared with the blank control group, the relative abundations of N′-cholinylglycine, quinoline-4, 8-diol, tryptophan, indole-3-acetamide, 5-hydroxyindoleacetic acid, estradiol 17β-3-sulfate, cortisone, N′-formylkynurenine and taurocholic acid in serum in the model group decreased to varions degrees. The relative abundance of 19-hydroxyenedione, deoxycorticosterone and 11b-hydroxyprogesterone increased to different degrees. After the intervention of 5% Scutellaria baicalensis water extract, the relative abundance of these 12 metabolites in the control group was significantly or extremely significantly reversed（P＜ 0.05 or P＜0.01, P＜0.001）. This process involved tryptophan metabolism, steroid hormone biosynthesis, cholesterol metabolism, bile secretion and other metabolic pathways. These results indicated that 5% water extract of scutellaria baicalensis could effectively reduce the clinical symptoms and mortality of Tilapia infected with Streptococcus agalactis. Scutellaria baicalensis played a role in the prevention and treatment of Streptococcus agalactiae disease of Tilapia by regulating metabolism of tryptophan, biosynthesis of steroid hormones, cholesterol metabolism, bile secretion and other metabolic pathways.
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