Effects of adding β-glucan to feed on the growth performance, immune indexes, transcriptome and intestinal flora of Epinephelus fuscoguttatus ♀×Epinephelus lanceolatus ♂

【Objective】To clarify the immune response mechanism induced by β-glucan in Epinephelus fuscoguttatus ♀×Epinephelus lanceolatus ♂，so as to provide a theoretical basis for the application of β-glucan as an immune enhancer in aquafeed.【Method】Taking E. fuscoguttatus ♀×E. lanceolatus ♂ as the research object，the breeding experiment was carried out by adding 100 mg/kg β-glucan to the feed. After 8 weeks of feeding，samples were collected for growth performance，serum biochemical indicators，transcriptome and intestinal bacteria community analysis.【Result】Adding β-glucan to feed significantly increased the final body mass，weight gain rate and specific growth rate（P<0.05，the same below）， and decreased the liver-body weight ratio. Total protein content in serum，the activities of catalase，lysozyme，pepsin and intestinal lipase in the grouper with β-glucan were significantly higher than those in the control group，and total triglyceride content，total cholesterol content，glutamic pyruvic transaminase activity and malondialdehyde content decreased significantly. Transcriptome sequencing results showed that 79291 unigenes were assembled，with an average length of 941 bp；according to the screening condition of FDR ≤0.05 and |log₂ FC（Sample2/Sample1）|≥1，a total of 8014 differentially expressed genes（DEGs）were obtained（4990 up-regulated genes and 3024 down-regulated genes）；648 DEGs were found to be enriched on 175 KEEG signaling pathways by KEGG signal pathway enrichment analysis，and there were 16 pathways associated with immune function，involving 139 significantly differential genes. The results of HiSeq PE250 high-throughput sequencing analysis showed that the number of intestinal bacteria community OUTs，ACE index and Shannon index in the β-glucan-adding group of E. fuscoguttatus ♀×E. lanceolatus ♂ were significantly higher than those in control group. There was no significant difference in Chao1 index and Simpson index（P>0.05）. At the phylum level， Proteobacteria，Actinomycetes，Bacteroidetes，Cyanobacteria and Firmicutes were the dominant bacteria communities for E. fuscoguttatus ♀×E. lanceolatus ♂. In the β-glucan-adding group；the relative abundances of Proteobacteria and Cyanobacteria were significantly lower than those in control group. while the relative abundances of Firmicutes and Bacteroidetes in β-glucan group were significantly higher than those in control group（P<0.01，the same below）. At the genus level，the dominant bacteria communities were Photobacterium，Sphingosphingomonas，Halomonas，Rhodoplanes and Achromobacter；the relative abundance of Photobacterium in β-glucan-adding group was significantly lower than that in the control group and the relative abundance of Halomonas was significantly lower than that of the control group.【Conclusion】β-glucan can effectively enhance the antioxidant ability of E. fuscoguttatus ♀× E. lanceolatus ♂ and improve the species richness and diversity of its intestinal bacteria communities to resist the reproduction of external harmful bacteria. At the same time，β-glucan can improve immunity of E. fuscoguttatus ♀×E. lanceolatus ♂ to resist diseases and inflammation. That is，β-glucan has positive effect on growth and immunity enhancement on E. fuscoguttatus ♀× E. lanceolatus ♂， and it can be popularized and applied as a new type of immunity enhancer.