Effects of prebiotic mixtures on growth performance, intestinal microbiota and immune response in juvenile chu's croaker, Nibea coibor

Highlights

• Two prebiotic mixtures (IG and GS) improved the growth performance (i.e. WG, SGR) of fish.

• Prebiotic mixtures could modulate the composition of intestinal microbiota and SCFAs production.

• Prebiotics activated the serum complement and cytokine levels, lysozyme activity and intestinal antioxidant capacity.

Abstract

Prebiotics has been known to be growth promoter and immunostimulant in aquatic animals. In this study, we investigated the effects of prebiotics on growth performance, intestinal microbiota, short-chain fatty acids (SCFAs) production and immune response of the marine fish, juvenile chu's croaker (Nibea coibor). The fish were fed IG (including 0.5% inulin and 0.5% GOS), GS (0.5% GOS and 0.5% D-sorbitol), IGS (0.33% inulin, 0.33% GOS and 0.33% D-sorbitol) or control diets for 8 weeks. The results showed that the growth performance of the fish was promoted by IG and GS, but not by IGS. The intestinal microbiota in NDC (non-digestible carbohydrates, NDC)-supplemented groups was clearly separated from that of the control, and the highest Shannon and Simpson diversity indices were observed in the IGS group. In the intestine of the croaker, Proteobacteria, Firmicutes, and Bacteroidetes were dominant; among them, 24 taxa revealed a significant difference among groups. Most of these bacteria are able to produce SCFAs, which were significantly increased in all NDC-supplemented groups. Moreover, NDCs were found to activate the immune system of the fish by modulating the serum complements, cytokine levels, lysozyme activities and antioxidant capacity. Furthermore, the results of this study revealed correlations among intestinal microbiota, SCFAs production, innate immunity, antioxidant capacity and digestive enzymes in the croaker fed NDCs. Taken together, our results demonstrated that NDC mixtures might promote growth performance, antioxidant capacity and immune responses of the croaker through modulating the composition of intestinal microbiota and the subsequent SCFAs production, which suggest that NDCs were efficient feed additives for marine fish.

Introduction

Aquaculture is one of the fastest growing food production industries and plays a significant role in meeting global protein needs for human [1]. The greater public awareness for healthy food has led to the banning of antibiotics used in aquaculture and an increased interest in the functional feed additives. Prebiotic is a promising feed additive, which is defined as “an effective substrate that is selectively utilized by host microorganisms conferring a health benefit” [2]. Since then, several non-digestible carbohydrates (NDCs), including oligosaccharides, plant cell wall polysaccharides, resistant starches and sugar alcohols were considered as potential prebiotics [3]. Previous studies revealed that prebiotics has beneficial effects on growth performance, feed utilization, carcass composition, intestine microbiota, SCFA levels, immunity system, and disease resistance in aquatic animals [[4], [5], [6]]. For example, galacto-oligosaccharides (GOS) has been widely used in various aquatic animals, such as Asian snakehead (Channa striata) [7,8], Caspian roach (Rutilus rutilus) [9], common carp (Cyprinus carpio) [10,11], hybrid striped bass (Morone chrysops × Morone saxatilis) [12] and red drum (Sciaenops ocellatus) [13]. Also, inulin has benefits in Asian seabass (Lates calcarifer) [14], common carp (C. carpio) [11], Gilthead seabream (Sparus aurata) [15], Nile tilapia (Oreochromis niloticus) [[16], [17], [18]], rainbow trout (Oncorhynchus mykiss) [19] and white shrimp (Litopenaeus vannamei) [20,21]. Although D-sorbitol is not widely used in aquaculture, it has been demonstrated to augment the concentrations of acetate, propionate and butyrate in human fecal cultures in vitro [22].

The suggested mechanism action of prebiotics involves changes in bacterial communities, with an increased account of the beneficial bacteria which produce inhibitory compounds, compete with pathogens for nutrients or adhesion sites, inhibit virulent gene expression or disrupt quorum sensing in aquatic animals [4,23]. Other mechanisms have been proposed to be related to the induction of the end-products of bacterial fermentation, or interactions between prebiotics and pattern recognition receptors of the hosts [4,24,25]. Microbial fermentation of NDCs mainly stimulates the production of SCFAs, mainly acetate, propionate, and butyrate, which play important roles in keeping the microbial homeostasis and host health [26]. In aquatic animals, it has been reported that SCFAs and their salts serve as a source of energy, promote the proliferation of intestinal epithelial cells, exert the antimicrobial activity by lowering intestinal pH, modulate the composition of intestinal microbiota, and enhance the immune response by increasing the levels of immune components and regulating the expression of immune-related genes [4,[27], [28], [29], [30], [31], [32]]. In grass carp, it has been shown that sodium butyrate enhanced the intestinal immune function of fish by elevating lysozyme and acid phosphatase activities, increasing the contents of complement C3, C4 and immunoglobulin M, down-regulating the pro-inflammatory cytokines (such as TNF-α, IL-1β, IL-6, and IL-8), and up-regulating the anti-inflammatory cytokines (such as IL-10, TGF-β1 and TGF-β2). In addition, sodium butyrate increased enteritis resistance of fish challenged with Aeromonas hydrophila [33]. However, the mechanisms of action of SCFA in fish immune system are poorly understood and need extensive research.

Chu's croaker, Nibea coibor, a member of the family Sciaenidae, is an economically important marine fish widely distributed in Japan, Korea, India and Southeast China [34]. In our previous works, it has been found the relationship between potential SCFA-producing bacteria and diets in the intestine of Nibea [35], and the ability of three NDCs (inulin, GOS, and D-sorbitol) to promote butyrate formation in in vitro intestinal cultures of Nibea (unpublished data). In this study, these NDCs were used to assess their effects on growth performance, intestinal microbiota, SCFA levels, immune response and antioxidant defense in juvenile chu's croaker. The findings of this study provide a better understanding on the beneficial stimulation of selective prebiotics on the chu's croaker gut microbes (including SCFA-producing bacteria), as well as the effects of microbes and SCFAs on the growth performance and immunity of hosts.