Dietary Mannan Oligosaccharides Enhance the Non-Specific Immunity, Intestinal Health, and Resistance Capacity of Juvenile Blunt Snout Bream (Megalobrama amblycephala) Against Aeromonas hydrophila

Abstract

Mannan oligosaccharides (MOS) have been studied and applied as a feed additive, whereas their regulation on the growth performance and immunity of aquatic animals lacks consensus. Furthermore, their immunoprotective effects on the freshwater fish Megalobrama amblycephala have not been sufficiently studied. Thus, we investigated the effects of dietary MOS of 0, 200, and 400 mg/kg on the growth performance, non-specific immunity, intestinal health, and resistance to Aeromonas hydrophila infection in juvenile M. amblycephala. The results showed that the weight gain rate of juvenile M. amblycephala was not significantly different after 8 weeks of feeding, whereas the feed conversion ratio decreased in the MOS group of 400 mg/kg. Moreover, dietary MOS increased the survival rate of juvenile M. amblycephala upon infection, which may be attributed to enhanced host immunity. For instance, dietary MOS increase host bactericidal and antioxidative abilities by regulating the activities of hepatic antimicrobial and antioxidant enzymes. In addition, MOS supplementation increased the number of intestinal goblet cells, and the intestine was protected from necrosis of the intestinal folds and disruption of the microvilli and junctional complexes, thus maintaining the stability of the intestinal epithelial barrier. The expression levels of M. amblycephala immune and tight junction-related genes increased after feeding dietary MOS for 8 weeks. However, the upregulated expression of immune and tight junction-related genes in the MOS supplemental groups was not as notable as that in the control group postinfection. Therefore, MOS supplementation might suppress the damage caused by excessive intestinal inflammation. Furthermore, dietary MOS affected the richness and composition of the gut microbiota, which improved the gut health of juvenile M. amblycephala by increasing the relative abundance of beneficial gut microbiota. Briefly, dietary MOS exhibited significant immune protective effects to juvenile M. amblycephala, which is a functional feed additive and immunostimulant.

Keywords: Megalobrama amblycephala; immunoprotective effects; intestinal health; mannan oligosaccharides; non-specific immunity.

Graphical Abstract

Figure 1

Dietary MOS decreased the mortality of juvenile M. amblycephala post–bacterial infection. Different letters indicated significant differences among groups (P < 0.05).

Figure 2

Effects of MOS supplementation on the activities of hepatic antimicrobial and antioxidant enzymes of juvenile M. amblycephala. (A–F) showed the enzymes activity of ACP, AKP, LZM, GST, SOD, and CAT, respectively. The asterisks indicated statistically significant differences among different groups at a certain time point (P < 0.05).

Figure 3

Effects of MOS supplementation on the intestinal histological structures of juvenile M. amblycephala by H&E staining. (A–C) Mid-intestine sections of control, MOS200, and MOS400 groups at 0 hpi, respectively. (D–F) Sections at 12 hpi. (G–I) Sections at 24 hpi. (J–L) Sections at 72 hpi. The pathological symptoms were marked with triangle. Scale bars represented 50 µm (400×).

Figure 4

Effects of MOS supplementation on the numbers of intestinal goblet cells by AB-PAS staining. (A–C) Mid-intestine sections of control, MOS200, and MOS400 groups at 0 hpi, respectively. (D–F) Sections at 12 hpi. (G–I) Sections at 24 hpi. (J–L) Sections at 72 hpi. Goblet cells were marked with triangle. Scale bars represented 50 µm (200×).

Figure 5

Effects of MOS supplementation on the intestinal ultrastructure of M. amblycephala by TEM assay. (A–C) Mid-intestine sections of control, MOS200, and MOS400 groups at 0 hpi, respectively. (D–F) Sections at 24 hpi. G, goblet cell. Scale bars represented 2 µm (8,000×).

Figure 6

Expression patterns of M. amblycephala intestinal immune and tight junction related genes in the three groups upon infection. The detected genes including MR (A), p38α (B), p38β (C), PKC (D), TNFα (E), IL-1β (F), IL-6 (G), CXCL8 (H), Muc2 (I), Occludin (J), Claudin-1 (K), and ZO-1 (L), and GAPDH was selected as the reference gene. Data were shown as mean ± SE, differences were determined by one-way analysis of variance (ANOVA). The asterisks indicated statistically significant differences among different groups at a certain time point (P < 0.05).

Figure 7

Venn diagram analysis of OTU numbers in the control and MOS400 groups.

Figure 8

Comparison of gut microbial composition between the control and MOS400 groups with weighted UniFrac PCoA analysis (A) and non-metric multidimensional scaling (NMDS) diagram (B).

Figure 9

Dietary MOS affected the gut microbial composition of juvenile M. amblycephala. Relative abundance of gut microbiota at phylum (A), genus (B), and species (C) levels.

Figure 10

Cladogram revealing the polygenetic distribution of bacterial lineages associated with different groups. Different colors indicated different groups; nodes in red or green represented the microbiome that played important roles in the control or MOS400 groups, whereas yellow nodes indicating the microbiome were not vital in both groups. The circles were in order of phylum, class, order, family, and genus levels from inside to outside.