1. Introduction
Because of the potential effects of environmental stressors, the weaning period is a life stage that can substantially affect pig production. In this important period, weaned pigs must rapidly adapt to nutritional, immunological, and psychological changes [
1]. Indeed, piglets can suffer from symptoms such as reduced feed intake, diarrhea, and villous atrophy of the small intestine that negatively affect digestion and absorption of nutrients during the early weaning stage [
2]. In addition, weaning stress can induce changes in the intestinal structure and function as well as intestinal microbiota of piglets. Moreover, intestinal barrier function disorders caused by weaning stress can disrupt the intestinal microecological balance of piglets, potentially leading to the invasion of pathogens in the intestine and various disorders such as inflammation and diarrhea [
3].
Including feed additives in the diet of piglets is a practical approach to alleviating intestinal barrier dysfunction during the weaning period [
4]. Yeast or yeast-based products have been reported beneficial prebiotic effects on growth performance, intestinal morphology, and immunity of pigs, which can preventing various diseases [
4,
5]. Yeast cell wall (YCW) is thought to be effective due to its components, which consists of specific sugar types such as mannan oligosaccharides, α-D-mannans, and β-D-glucans [
6]. Previous studies have reported that yeast-based products may be an effective alternative to antibiotics as without differences in growth performance, nutrient digestibility, and intestinal morphology in piglets [
7,
8]. Indeed, the addition of YCW or YCW-based products in weaned pig diets has been shown to increase villus heights and villus-to-crypt ratio [
7,
9]. In addition, prebiotics derived from YCW can boost the production of immunoglobulins and antibodies, which play a crucial role in the immunity of weaned pigs [
10,
11]. However, the physiological correlation between dietary YCW as prebiotic effects and pig’s gut health in a weaning stress condition needs to be further supported. The objective of this study, therefore, was to investigate the effects of YCW on health of weaned pigs, focusing on their growth performance, nutrient digestibility, immune responses, and intestinal health and microbiota.
4. Discussion
In the present study, dietary YCW did not affect ADG, ADFI, and G:F of weaned pigs for the first 2 weeks after weaning. In addition, the diarrhea frequency was increased in the CON group for 2 weeks after weaning, which may be related to weaning events [
2,
3,
22]. However, dietary YCW tended to increase ADG in weaned pigs when considered over the entire experimental period, suggesting that YCW supplementation may improve growth performance. Moreover, YCW supplementation reduced the diarrhea frequency of weaned pigs. These observations are in agreement with those of previous studies, in which the improved growth performance, nutrient absorption, and frequency of diarrhea are related to feed ingredients, intestinal morphology, and gut microbiota [
4,
5]. Furthermore, another study showed supplementation of yeast-based products increased the growth performance of weaned pigs [
23]. Interestingly, the observations in the present study are consistent with those of a previous study, in which dietary yeast product supplementation did not affect ADG for the first 3 weeks after weaning but increased the ADG of weaned pigs following 6 weeks of consumption [
24]. Therefore, we postulate that the ingestion of YCW over relatively long periods may have positive effects on the intestinal morphology and, thereby, increase nutrient digestibility and improve the growth performance of weaned pigs.
The AID of DM and ATTD of energy of weaned pigs were increased with YCW supplementation in the present study. Similarly, a previous study showed dietary YCW improved the growth performance and nutrient digestibility of weaned pigs [
25]. Intestinal morphology, including villus height, width, and area, as well as crypt depth and the goblet cell counts, is generally associated with nutrient digestion and absorption [
25,
26], and it is known that yeast-based products improve nutrient digestibility due to their beneficial effects on intestinal morphology and function [
25,
26]. The cell wall of yeast is composed of mannoproteins, β-(1,3) or (1,6)-D-glucans, chitin, and glycophospholipid proteins related to the plasma membrane [
8]. The YCW components include specific sugar types, such as β-D-glucans, α-D-mannans, and mannan oligosaccharides, which have prebiotic effects [
8]. In the present study, villus width and area and VH:CD were increased in the ileum and duodenum, while the goblet cell counts increased in the duodenum and jejunum as a result of YCW supplementation: these results agree with previous observations, in which prebiotics derived from YCW increased the proliferation of intestinal cells and the concentration of glycoconjugates in the mucins [
7,
9].
The tight junction is a crucial structure related to the epithelial barrier function of the intestinal tract. The physical barrier function in the intestine is associated with the tight junction structure between intestinal epithelial cells. Therefore, the health of the intestinal tight junction is crucial to maintaining polarity in intestinal epithelial cells and preventing a material spill in the epithelial cell gap. Claudin and occludin members of the tetraspan family related to transmembrane proteins form part of the gut structure and play roles in the epithelial barrier function of the intestine [
27]. Results from a previous experiment indicate that the gene expression of claudin and occludin proteins could be an indicator of epithelial barrier function in the intestine [
27]. In the present experiment, we observed that dietary YCW supplementation significantly increased the gene expression of claudin, occludin, and mucin proteins in the ileum of weaned pigs. This is consistent with the results of previous studies, which showed that the supplementation of yeast-based products upregulated the gene expression of tight junction proteins, including claudin and occludin in the intestinal mucosa [
9] and improved barrier function via the increased thickness of the mucosal layer and number of mucosal macrophages [
28].
We performed 16S rRNA gene analysis to evaluate the effects of YCW on fecal microbiota of weaned pigs. During weaning period, an alteration of dynamic microenvironment occurs in the piglet’s intestinal tract with the changes in diets and age [
29]. In the present study, YCW supplementation increased genera
Prevotella and
Roseburia. This is consistent with the previous studies that yeast and its derivatives, as probiotic effects, can modify the community of gut microbiota and reduce diarrhea, which improve the health of early-weaned piglets [
30].
Prevotella strains are the most dominant bacterial community at genus level [
31]. In addition,
Prevotella and
Roseburia strains are related to the production of short chain fatty acids (SCFAs) by the fermentation of indigestible polysaccharides in the intestine [
32]. These
Prevotella and
Roseburia strains promote nutrient digestibility; thus, YCW supplementation increases the nutrient digestibility of weaned pigs possibly through the intestinal microbiota, especially
Prevotella and
Roseburia. It is known that the SCFAs, such as acetate, propionate, and butyrate, regulate the barrier function and immune system and prevent the establishment of pathogenic communities in the gut [
32]. Therefore, the observation of microbiota in the present study supports that YCW supplementation could improve the nutrient digestibility with the enhancement of epithelial barrier and immune function in weaned pigs by increasing the relative abundance of
Prevotella and
Roseburia.
It has been reported that stress factors during the weaning period induce inflammation in the intestine and serum of weaned pigs, which leads to an increase in typical pro-inflammatory cytokines, such as TNF-α, IFN-γ, IL-β1, IL-6, and TGF-β1 [
3]. In addition, both pro- and anti-inflammatory cytokines play a key role in the regulation of immune responses and intestinal barrier function [
33]. In the present study, YCW supplementation increased the gene expression of some pro-inflammatory cytokines, such as IL-β1 and IL-6, and reduced that of TNF-α; furthermore, the gene expression of anti-inflammatory cytokine, INF-γ, was upregulated in the ileum. Moreover, it was observed that serum TNF-α and IL-β1 concentrations were decreased on day 7 and 14, respectively, whereas serum TGF-β1 concentration was increased on day 14 in weaned pigs fed YCW. The TGF-β1 considered as an indicator of anti-inflammatory cytokines in the present study generally functions as both an immune-suppressor and -enhancer that modulate the proliferation and differentiation of T and B cells. On the other hand, the TNF-α and IL-6, which are typical pro-inflammatory cytokines, stimulate macrophages to induce inflammatory responses and regulate immune responses [
34]. These results suggest that YCW supplementation in weaning diets is beneficial for immune-modulation. Previous studies are in agreement with the concern [
10,
11]: YCW products stimulated the immune system to increase antibody production because the β-D-glucans from YCW bind to receptors causing cytokine cascades and improved macrophage function [
35]. A previous study demonstrated that the α-D-mannans in YCW bind to mannose-specific receptors and, thereby, prevent the adhesion of pathogens to the intestinal barrier [
10]. Therefore, the present study indicates that the dietary YCW supplementation could attenuate the inflammation caused by weaning stress.