Abstract
Diseases of crucian carp (Carassius auratus) are closely related to intestinal parameters. Enterococcus faecalis 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 E. faecalis 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 E. faecalis at 105 CFU/g (EF1), 106 CFU/g (EF2), or 107 CFU/g (EF3) for 28 days. The results showed that supplementation of E. faecalis significantly improved the weight gain ratio (WGR) and the specific growth rate (SGR) compared with control group (p < 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 (p < 0.05), and serum alkaline phosphatase was significantly higher in all experimental groups (p < 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 Aeromonas and Acinetobacter decreased while Cetobacterium increased in all experimental groups, with the greatest changes in EF2. Challenge tests showed that fish fed E. faecalis were more resistant to Aeromonas veronii (p < 0.05). In conclusion, dietary E. faecalis YFI-G720 at 106 CFU/g can improve the health status, immune parameters, intestinal microbiota composition, and disease resistance of crucian carp.
Highlights
Crucian carp (Carassius auratus), an omnivorous and bottom-feeding freshwater fish, is one of the main aquaculture species in China [1,2]
Experimental crucian carp were obtained from a local farm in Wuhan, Hubei, China, and we randomly sampled for parasite observation, bacterial isolation, and Cyprinid herpesvirus 2 (CyHV-2) detection, and the results showed negative
After the feeding trial lasting 28 days, the survival rates of the four crucian carp groups were >95% (Table 2), and there was no significant difference between the groups
Summary
Crucian carp (Carassius auratus), an omnivorous and bottom-feeding freshwater fish, is one of the main aquaculture species in China [1,2]. With the increasing development and intensification of aquaculture, diseases caused by pathogens affecting this species are gradually increasing, such as Aeromonas veronii, Aeromonas hydrophila, and Cyprinid herpesvirus 2 (CyHV-2) [5,6,7]. These pathogens mainly spread to other tissues and organs after infecting the intestinal tract. For the crucian carp industry, it is important to maintaining a balance of intestinal microbiota via good health management
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