Effects of Dietary Yeast Culture Supplementation on Growth Performance, Digestive Function, and Intestinal Health of Largemouth Bass Micropterus salmoides

This study was performed to investigate the effects of dietary yeast culture (YC) supplementation on growth performance, digestive function, intestinal inflammatory response, and microbiota composition of largemouth bass <i>Micropterus salmoides</i> (LMB). Six diets were formulated with graded levels of YC (0, 5, 10, 15, 20, and 30 g/kg), referred to as CON, YC5, YC10, YC15, YC20, and YC30, respectively. Each diet was assigned to four replicate tanks of LMB juveniles (initial body weight 8.11 ± 0.05 g) with twenty fish per tank. After an 8-week feeding trial, final body weight and specific growth rate showed an increasing trend with 5~20 g/kg YC and reached a maximum at 15 g/kg YC. Feeding ratio decreased, but feed efficiency ratio (FER) improved in response to dietary YC inclusion, and FER was higher in the YC10 fish than in the YC5, YC20, and YC30 fish. Proximate composition (moisture, protein, and lipid) of the whole fish was not affected by dietary YC levels. The activities of intestinal lipase and trypsin were higher in the YC10 fish, while the relative expression of interleukin-8 (<i>il-8</i>) and <i>il-1β</i> was downregulated in the hindgut of the YC15 fish compared with the CON fish. Histological examination showed that the villus height of the midgut, together with goblet cell density of the foregut and midgut, was higher in the YC10 and YC30 fish than in the CON fish. 16S rRNA sequencing showed that <i>Proteobacteria</i>, <i>Fusobacteria,</i> and <i>Firmicutes</i> dominated the intestinal microbiota in LMB. The decrease in harmful <i>Mycoplasma</i> accounted for the dramatic change in <i>Firmicutes</i> abundance, while the increase in <i>Cetobacterium</i> (specifically <i>C. somerae</i>) accounted for the change in <i>Fusobacteria</i> abundance in the gut of the YC10 and YC30 fish compared with the CON fish. The increase in the beneficial <i>Endozoicomonas</i> was the main reason for the change in Proteobacteria abundance in the intestine of the YC30 fish as compared with the CON fish. Taken together, the alteration of intestinal microbiota composition contributed to the improved digestive function and feed utilization in LMB fed YC-supplemented diets. Based on growth performance, the optimal YC level in the diet for LMB was 15 g/kg.