Dietary methanotroph bacteria meal alleviates soybean meal-induced enteritis by improving immune tolerance and intestinal flora profile of juvenile turbot (Scophthalmus maximus L.)

An 8 -week growth trial was performed to investigate the protective effects of methanotroph bacteria meal (MBM) produced from methane against soybean meal -induced enteritis (SBMIE) in juvenile turbot (Scophthalmus maximus L.). Five isonitrogenous and isolipidic diets were formulated: fishmeal -based diet (FM, the control group); FM with approximate 50% of fishmeal substituted by 399.4 g/kg soybean meal (SBM); SBM supplemented with 63.6, 127.2 and 190.8 g/kg MBM (named MBM1, MBM2 and MBM3), each diet was randomly assigned to triplicate fibreglass tanks. Results showed that fish fed with SBM exhibited enteritis, identified by reduced relative weight of intestine (RWI), as well as expanded lamina propria width and up -regulated gene expression of pro -inflammatory cytokines (tnf-alpha, il-6 and il-8) in intestine. While the above symptoms were reversed when diet SBM supplemented with MBM at the levels of 63.6 and 127.2 g/kg, as well as characterized by up -regulated gene expression of anti-inflammatory cytokines (tgf-beta and il-10) and tight junction protein (claudin3, claudin4 and claudin7) in intestine. Intestinal transcriptome analysis showed that the differentially expressed genes between groups FM and SBM predominantly enriched in the JAK-STAT signaling pathway, and the enrichment of differentially expressed genes between groups SBM and SBM supplemented with 63.6 g/kg MBM was in the inflammatory bowel disease (IBD) and JAK-STAT signaling pathway. To be specific, the expression of jak1, jak2b, stat1 and stat5a was significantly up -regulated when fish fed with SBM, suggested the activation of JAK-STAT signaling pathway, while the expression of these above genes was depressed by providing MBM to diet SBM, and the gene expression of toll -like receptors tlr2 and tlr5b showed a similar pattern. Moreover, intestinal flora analysis showed that community richness and abundance of beneficial bacteria (Cetobacterium and acillus_coagulans) were improved when fish fed with SBM supplemented with 63.6 g/kg MBM. Overall, methanotroph bacteria meal may alleviate SBMIE by regulating the expression of tight junction protein, toll -like receptors and JAK-STAT signaling pathway, as well as improving intestinal flora profile, which would be beneficial for enhancing the immune tolerance and utilization efficiency of turbot to dietary soybean meal.