Evaluation of the preventive effect of phage cocktails on turbot ascites and its influence on main physiological indicators

Abstract

Edwardsiella tarda and Vibrio harveyi are the main pathogens causing ascites in turbot, which severely restricts the development of turbot farming. This study evaluated the effect of feeding phage cocktail on the prevention of ascites in turbot. The turbot was challenged by bacterial suspension comprising 50 μL of E. tarda ET9 at a concentration of 5.99 × 106 CFU/fish and 50 μL of V. harveyi VH5 at a concentration of 7.86 × 107 CFU/fish after they were fed with phage cocktails for 7 days. The cumulative survival rate of negative control group was 100%, and that of positive control group decreased to 7%, while that of phage-treated group was 60%, 83% and 87% when multiplicity of infection (MOI) was equal to 1, 10 and 100 respectively after 15 days of challenge. The pathological section showed that only the liver and intestines of turbot in the positive control group had lesions, while that of the phage-treated group had no obvious symptom. The contents of white blood cells, red blood cell distribution width and platelets in the blood of the positive control group were significantly higher than those of the negative control group and the phage-treated group (P < 0.05). In contrast, there was no significant difference between the negative control group and the phage-treated group (P > 0.05). Only the levels of Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), Total protein (TP), Globulin (GLO), Total bile acid (TBA), Cholinesterase (CHE), Creatinine (CREA), Uric acid (UA), Glucose (GLU), Total cholesterol (CHOL), Low density lipoprotein (LDL), Creatine kinase (CK) and Creatine kinase isoenzymes (CKMB) in the serum of turbot in the positive control group were significantly different from those in the negative control group and the phage-treated group (P < 0.05), but there was no significant difference between the negative control group and the phage-treated group (P > 0.05). The transcription levels of 1L-1β, 1L-8, TNF-α and C 3 in the head kidney and spleen of turbot fed with phage cocktails with MOI = 10 were significantly lower than those of the positive control group after 24 h of challenge (P < 0.05). The intestinal flora of turbot in phage-treated group and positive control group was analyzed by high-throughput sequencing. The results showed that the abundance of Vibrio species and Edwardsiella species in turbot's intestine was significantly reduced by feeding with phage cocktails. These results indicate that feeding phage cocktails can effectively prevent bacterial ascites on turbot.