Effects of Dietary Yeast Culture on Growth, Intestinal health and Immunity of Micropterus salmoides

Yeast culture is a micro-ecological product formed by anaerobic fermentation of yeast, containing a variety of active substances. The present study was to investigate the effects of yeast culture supplementation on growth performance, intestinal health, immunity and cumulative survival rate after bacteria challenge in largemouth bass (Micropterus salmoides). 1 g/kg (S1 diet) and 3g/kg (S2 diet) of yeast culture supplementation were added to the control diet (Con diet), respectively. The three experimental diets were formulated with the same nutritional contents of protein (52%) and lipid (8.5%). The growth experiment was conducted in the pond cage (Shishou, Hubei, cage with the size of 2m×2m×2m). The 360 largemouth bass with the similar initial body weight of (32.37±0.15) g were divided into 3 dietary treatments, each treatment had 3 replicates cages, and 40 fishes were stocked in each cage. The growth experiment period was 11 weeks. At the end of the growth experiment, the samples were taken from each cages 7 h after the last feeding. The results showed that the final body weight (FBW), weight gain rate (WGR) and specific growth rate (SGR) of supplemented yeast culture groups had no significant difference compared with the control group (P>0.05). The condition factor (CF), hepatosomatic index (HSI) of supplemented yeast culture groups had no significant difference compared with the control group (P>0.05), while S2 group had the lowest value of HSI. In terms of intestinal structure, the villus length of the S2 group was significantly increased compared with the control group, and the depth of crypts was significantly decreased compared with the control group(P<0.05). For intestinal microbes, S1 group and S2 group both reduced the contents of Cyanobacteria and Staphylococcus in the intestine and increased the intestinal content of Clostridiales compared with the control group. Compared to the control group, S2 group significantly increased (P<0.05) the activities of head kidney superoxide dismutase (SOD), catalase (CAT), alkaline phosphatase (AKP) and lysozyme (LZM). The contents of head kidney malondialdehyde (MDA) in yeast culture supplementation groups had no significant difference compared with the control group (P>0.05). Compared to the control group, the S1 group also significantly increased the activity of plasma SOD (P<0.05). Compared to the control group, S2 group significantly reduced the plasma malondialdehyde (MDA) content (P<0.05). Compared to the control group, the yeast culture supplementation group increased the plasma lysozyme activities, but there was no significant difference (P>0.05). The cumulative survival rate of S1 and S2 groups was significantly higher than that of the control group after 252 h changed with Aeromonas hydrophila (P<0.05). The present results indicated that dietary supplementation of 0.1-0.3% yeast culture had positive effects on intestinal health, antioxidant capacity, immunity and disease resistance in largemouth bass.