The Effects of Sodium Propionate Supplementation in the Diet with High Soybean Meal on Growth Performance, Intestinal Health, and Immune Resistance to Bacterial Infection in Turbot (Scophthalmus maximus L.)

Abstract

Short-chain fatty acids (SCFAs) are the products of the microbial fermentation of dietary fiber in the intestine. Acetate, propionate, and butyrate are the most abundant SCFA metabolites and play an important role in maintaining host health. This study was aimed at investigating the effects of sodium propionate (NaP) supplementation in the diet with a high proportion of soybean meal (SBM) on the growth, inflammatory status, and anti-infectious ability in juvenile turbot. Four experimental diets were designed: (1) fish meal- (FM-) based diet (control group), (2) SBM protein replacing 45% FM protein in the diet (high SBM group), (3) 0.5% NaP supplementation in the high SBM diet (high SBM+0.5% NaP group), and (4) 1.0% NaP supplementation in the high SBM diet (high SBM+1.0% NaP group). The results confirmed that the fish fed the high SBM diet for 8 weeks showed the decreased growth performance, the typical enteritis symptoms, and the increased mortality responding to Edwardsiella tarda (E. tarda) infection. However, 0.5% NaP supplementation in the high SBM diet promoted the growth performance of turbot and restored the activities of digestive enzymes in the intestine. Moreover, dietary NaP ameliorated the intestinal morphology, enhanced the expression of intestinal tight junction proteins, improved the antioxidant capacity, and suppressed the inflammatory status in turbot. Finally, the expression of antibacterial components and the resistance to bacterial infection were increased in NaP-fed turbot, especially in high SBM+1.0% NaP group. In conclusion, the supplementation of NaP in high SBM diet promotes the growth and health in turbot and provides a theoretical basis for the development of NaP as a functional additive in fish feed.

Figure 1

Effects of NaP on the activities of digestive enzymes in distal intestine. (a–c) After the feeding trial, the distal intestine of turbot in different groups was collected, and the activities of trypsin (a), α-amylase (b), and lipase (c) in the distal intestine were measured (n = 12). Different superscript letters indicated significant difference (P < 0.05).

Figure 2

Effects of NaP on intestinal histomorphology and intestinal mucosal barrier. (a) The intestine of turbot in different groups was collected and sectioned. After the fixation by H&E, the morphology of the distal intestines was observed. The images were representative of at least three independent experiments, and scale bar indicates 500 μm; (A) and (B) in the images indicate villus height and muscle layer thickness, respectively; lamina propria is indicated by arrows. (b–d) The micromorphology, including villus height (b), lamina propria (c), and muscle layer thickness (d) of the intestine, was evaluated (n = 6). (e–g) The gene expression of ZO-1 (e), occludin (f), and claudin (g) in the intestine was analyzed by qRT-PCR (n = 12). Different superscript letters indicated significant difference (P < 0.05).

Figure 3

Effects of NaP on the expression of inflammatory cytokines. (a–c) The gene expression of TNF-α, IL-1β, IL-6, IL-10, TGF-β in the distal intestine (a), liver (b), and spleen (c) of turbot in different groups was analyzed by qRT-PCR (n = 12). Different superscript letters indicated significant difference (P < 0.05).

Figure 4

Effects of NaP on antioxidant enzyme activities in serum. (a–c) The serum was obtained as described in Materials and Methods. The levels of T-AOC (a), MDA (b), and SOD (c) of juvenile turbot in different groups were measured (n = 9). Different superscript letters indicated significant difference (P < 0.05).

Figure 5

Effects of NaP on the anti-infectious ability in turbot after bacterial infection. After the feeding trial, a part of turbot were infected by i.p. injection with E. tarda (1 × 10⁷ CFU/fish). (a) The mortality of the turbot was recorded every 6 hours (n = 10). (b) The viable bacteria in the spleen at 12 hpi were counted (n = 12). (c and d) The lysozyme activities in the distal intestine (c) and spleen (d) of infected turbot were measured (n = 12). Different superscript letters indicated significant difference (P < 0.05).

Figure 6

Effects of NaP on the bactericidal activity and the production of antibacterial effectors in HKMs. After the feeding trial, HKMs were isolated and cultured for 24 h. (a) E. tarda was added (cells: bacteria =1 : 1) and coincubated for 2 h. The bactericidal activity of HKMs was assessed (n = 6). (b and c) The gene expression of lysozyme (b) and enzyme activity (c) in the cells were measured (n = 6). (d and e) E. tarda was added (cells: bacteria =1 : 1) and coincubated for 2 h; the production of ROS (d) and NO (e) in the cells was analyzed (n = 6). Different superscript letters indicated significant difference (P < 0.05).