| Summary: | Diseases of crucian carp (<i>Carassius auratus</i>) are closely related to intestinal parameters. <i>Enterococcus faecalis</i> has strong colonization ability in the intestinal tract, and produces natural antibiotics, bacteriocin, and other bacteriostatic substances, which can effectively inhibit some pathogenic bacteria and improve the intestinal microenvironment. This study aimed to assess the effects of <i>E. faecalis</i> YFI-G720 which was isolated from the intestinal of crucian carp on the growth, immunity, intestinal health, and disease resistance of crucian carp. Fish (48.16 ± 0.55 g) were fed four diets, commercial diet or diet containing <i>E. faecalis</i> at 10<sup>5</sup> CFU/g (EF1), 10<sup>6</sup> CFU/g (EF2), or 10<sup>7</sup> CFU/g (EF3) for 28 days. The results showed that supplementation of <i>E. faecalis</i> significantly improved the weight gain ratio (WGR) and the specific growth rate (SGR) compared with control group (<i>p</i> < 0.05). Intestinal mucosal epithelial cells in EF2 were intact and normal, but there was obvious vacuolation in CG. Compared with CG, serum C3 and IgM in EF2 were significantly increased at the end of the experiment (<i>p</i> < 0.05), and serum alkaline phosphatase was significantly higher in all experimental groups (<i>p</i> < 0.05). Among studied immune-related genes, expression was detected by qPCR, C3, IgM, and IL-1βwere upregulated in all experimental groups to varying degrees from 14 days, with highest expression in EF2 at 28 days. Intestinal microbiota structure analyzed through high-throughput sequencing, and the results showed that the relative abundance of <i>Aeromonas</i> and <i>Acinetobacter</i> decreased while <i>Cetobacterium</i> increased in all experimental groups, with the greatest changes in EF2. Challenge tests showed that fish fed <i>E. faecalis</i> were more resistant to <i>Aeromonas veronii</i> (<i>p</i> < 0.05). In conclusion, dietary <i>E. faecalis</i> YFI-G720 at 10<sup>6</sup> CFU/g can improve the health status, immune parameters, intestinal microbiota composition, and disease resistance of crucian carp.
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