Effect of Dietary <i>Astragalus</i> Fermentation Products on Growth, Intestinal Microflora and Disease Resistance in Largemouth Bass <i>Micropterus salmoides</i>

Fermentation of Astragalus by Lactobacillus plantarum and Bacillus coagulans can increase the release of active components and degrade its macromolecular substances. This study investigated the effect of fermentation products (Astragalus + L. plantarum + B. coagulans, ALB) on largemouth bass. We specifically focused on growth performance, serum biochemical indices, intestinal microbial diversity, intestinal enzyme activity, immune gene expression and resistance to infections by Aeromonas hydrophila and largemouth bass ranavirus (LMBRaV). The largemouth bass were divided into five groups based on the amount of ALB added to the feed as following, (1) ALB0 (no ALB, ALB0.5 [0.5% addition of ALB], ALB1 [1% addition of ALB], ALB3 [3% addition of ALB], ALB5 [5% addition of ALB]). The feeding trial spanned 28 days. Comprehensively comparing the feeding results of different ALB concentration, the ALB0.5 group showed the best effect. The ALB0.5 group had significantly increased weight gain rate, alkaline phosphatase, total protein, albumin, digestive enzymes activities of lipase, trypsin and increased intestinal villi and thickness of muscularis propria. And it decreased feed conversion ratio, aspartate aminotransferase and alanine aminotransferase of largemouth bass. Furthermore, the ALB0.5 group improved the richness and diversity of the intestinal microbiota. Increased abundance of dominant phylum and genus in the intestine of largemouth bass included Fusobacteria and Cetobacterium, which promoted the growth and immune performance of largemouth bass. After infection with A. hydrophila and LMBRaV, the survival rates were higher in ALB addition experimental groups than in the ALB0 group, respectively. And the survival rate of ALB0.5 group was higher than other groups. Meanwhile, the ALB added to the feed could regulated the immune gene expression (Mx, IRF-3, TNF-α, IL-1β and IL-10), which also promoted the largemouth bass resistance to disease. In summary, adding 0.5% ALB to the diet of largemouth bass can boost its growth performance, immune genes expression, intestinal health and disease resistance.