A novel formula of herbal extracts regulates growth performance, antioxidant capacity, intestinal microbiota and resistance against Aeromonas veronii in largemouth bass (Micropterus salmoides)

In this study, a novel herbal extracts formula (HE) comprising Scutellaria baicalensis, Taraxacum mongolicum, Biancaea sappan and Pulsatilla chinensis extract with a weight ratio of 3.0: 1.8: 3.0: 1.7, was selected based on bacteriostatic test in vitro. Subsequently, a 30-day feeding trial was conducted to evaluate the effect of dietary HE (0, 0.05, 0.10, 0.50 and 1.00%, respectively) inclusion on the performance of largemouth bass (Micropterus salmoides). The results unveiled that dietary HE 0.10% and HE 0.50% inclusion notably increased the specific growth rate (SGR) and antioxidant enzyme (SOD and GPX) activities in the intestine and liver of fish (p < 0.05). Additionally, a significantly greater height of villi and width of villi were observed in the HE 0.50% and HE 0.10% groups, respectively (p < 0.05). Conversely, histological examination indicated that dietary HE1.00% inclusion led to discernible injury to the intestines, with certain villi displaying breakage and shedding. Hematological parameters including white blood cells (WBC) and red blood cells (RBC) were significantly increased, while the erythrocyte indices including average cellular hemoglobin (MCH), and average cellular hemoglobin concentration (MCHC) remarkably decreased in fish fed HE inclusion compared to the basal diet. The 16S rRNA sequence analysis demonstrated that Cetobacterium, Aeromonas and Plesiomonas were the three dominant species in the intestines regardless of diet. Additionally, the LEfSe analysis revealed that Aeromonas was the most affected taxon, which significantly decreased in the HE 0.10%, HE 0.50% and HE 1.00% groups (p < 0.05), while the Cetobacterium content remarkably increased in HE 0.10% and HE 0.50% groups (p < 0.05). Finally, the highest survival rate of fish infected with Aeromonas veronii HZ012 was observed in the group supplemented with 0.50% HE. Collectively, our study revealed that the supplementation of HE at a proportion of 0.10–0.50% could lead to a promotion of growth performance, maintenance of liver and intestinal health, as well as a protective effect against A. veronii infection in largemouth bass.