Yeast fermentation product is used widely in the livestock industry. The potential mechanisms of action of yeast products that maintains an advantageous intestinal environment  by regulating microbial ecology and elevates intestinal immunity through preventing pathogenic bacteria from intestinal epithelial cells, consequently improving growth performance have been reported [13, 14]. In this study, during the pre-challenge period, supplementation of 0.2% and 0.4% YFP had no significant effect on growth performance of piglets compared with the control. However, during the Salmonella typhimurium challenge period, 0.4% YFP significantly enhanced ADG compared with ST group. In general, the present study indicates that the relatively high dose of YFP offered protection against Salmonella typhimurium and that is beneficial to enhance growth performance of piglets during Salmonella typhimurium infection. The following discussion was focused on the 0.4% YFP effects after Salmonella typhimurium infection.
Intestinal morphology can be used as an indicator for intestinal health . To response inflammation induced by pathogens, deeper crypts exhibited faster cellular turnover allowing renewal of villus as needed . In the current study, dietary YFP supplementation improved intestinal histomorphology in the infectious piglets. Salmonella typhimurium challenge induced significantly deeper crypt and shorter villus in the duodenum, the same observations reported challenged with Salmonella typhimurium in piglets. In the challenged piglets, however, the dietary YFP supplementation remarkably increased villus height and decreased crypt depth, which contributed significantly to enhance V/C ratio, indicating the effective nutrient absorption capability and the decreased F/G.
Tight junctions among intestinal epithelial cells, which protect the body from intestinal pathogens, influence intestinal mucosal barrier function to a great extent . Occludin, as the major integral proteins, form the continuous tight junction strands . In the current study, YFP supplementation significantly increased occludin expression in duodenum, which is expected to improve intestinal mucosal barrier function. Our results demonstrated that YFP kept intestinal integrity and barrier function partially by elevating intestinal tight junction protein expression.
Currently, apoptosis is derived by the expression of Bax, Bcl-2 and caspase 3 . In this study, the mRNA abundances of Bax and caspase 3 were significantly up-regulated, and Bax protein expression level was also increased in the ST group, demonstrating that ST induced intestinal epithelial cell apoptosis, but both mRNA abundances and Bax protein expression were down-regulated with supplementation YFP. Our findings showed that YFP might protect intestinal barrier function by inhibiting apoptosis pathway.
LPS induces inflammation in pigs by stimulating the production of cytokines such as IL-1, IL-6 and TNFα, which are considered as endogenous mediators of inflammation . IL-10 is an important anti-inflammatory cytokine which is required for protection in the regulation of intestine homeostasis during host defense [21, 22]. HSP70 also plays a significant role on intestinal inflammation response. The concentrations of plasma IL-1β, IL-6 and IL-10 were measured as indicators of systemic pro- and anti- inflammation responses, respectively. In the present study, the increased plasma IL-1β and IL-6 concentrations indicated successful establishment of the Salmonella typhimurium challenge model. And Salmonella typhimurium caused an increased of intestinal HSP70 protein expression. Piglets receiving YFP had less concentration of plasma IL-1β and IL-6 and higher concentration of IL-10 than ST group, hence, implying that YFP lowered pro-inflammation of the immune system. The overproduction of pro-inflammatory cytokines is associated with anorexia, which may explain the observed reduction in ADG in Salmonella typhimurium-challenged piglets in our study [23, 24]. Meanwhile, the evidence that lower mRNA expression of IL-1β, IL-6 and TNFα, and higher IL-10 expression compared to the ST piglets in duodenum indicated that dietary supplementation with YFP contributed to beneficial immunoregulatory responses.
Toll like receptor 4 (TLR4) is the first identified of TLRs family, which recognizes the LPS component of Gram negative bacteria on the cell surface . The activation of TLR4 signaling pathway activates members of the MAPK family including p38 and ERK1/2, leading to numerous pro-inflammatory cytokine genes translation . TLR4 and MAPK signaling are of great significance in intestinal inflammation [27, 28]. In the present study, ST challenge increased mRNA expression of TLR4 and its downstream signals IRAK1, TRAF6 and NFκB. YFP supplementation decreased TLR4, MyD88, IRAK1, TRAF6 and NFκB mRNA expression and TLR4 protein, p38 phosphorylation and ERK1/2 phosphorylation. Duan et al.  demonstrated that mannan oligosaccharide (a component of YFP) supplementation could enhance intestinal mucosal immune competence and suppress intestinal inflammation by decreasing the contents of pro-inflammatory cytokines. Sun et al.  demonstrated that saccharomyces cerevisiae polysaccharide (a component of YFP) could decrease the pro-inflammation mediators of IL-1β and IL-6 at protein and mRNA levels by inhibiting MAPK activity in mice with DSS-induced colitis, which were consistent with our results.