The effects of Saccharomyces cerevisiae fermentation products (YFP) on growth performance and gastrointestinal (GIT) microbial ecology in 90 weanling pigs orally challenged with Escherichia coli K88+ (ETEC) were investigated. The YFP were an original YFP product (XPC) and a water-suspendable yeast fermentation prototype (WSYFP) from a commercial company. Treatments consisted of a negative control (NC, no in-feed or in-water additive), carbadox (AB, 55 mg of carbadox/kg of feed), XPC (in feed, 0.2%), and WSYFP (in water, 0.5, 1, or 2 g/pig per day), and each was allotted to 5 pens (3 pigs/pen). The diets met the 1998 NRC specifications. Pigs were acclimated to treatments for a 7-d period before an ETEC challenge. On d 8, blood was collected from pigs to determine the baseline packed cell volume (PCV) measurement, and pigs were orally challenged with ETEC. At various time points postchallenge, blood samples were taken, performance measures and fecal consistency scores were recorded, and gut digesta and tissue samples were taken to evaluate GIT morphology, microbial ecology, and metabolites. Preplanned contrasts were used for comparison. Pigs receiving YFP had greater ADFI than NC pigs on d 3 (424 vs. 378 g/d) and d 7 (506 vs. 458 g/d) postchallenge. This effect of YFP on ADFI was similar to that of AB on d 3, but pigs receiving AB ate more (576 vs. 506 g/d) at d 7 than pigs receiving YFP. Pigs exhibited reduced PCV upon ETEC challenge; however, pigs receiving additives sustained a greater PCV at 72 h compared with the NC group. Compared with the NC pigs, pigs receiving YFP showed a smaller number of ileal mucosa adherent ETEC and prevalence of the order Enterobacteriales in the ileal digesta, which corresponded to less (5.09 vs. 6.97 mg/ dL) colonic ammonia on d 7 post-challenge. Most of the indices for ileal digesta bacterial richness and diversity were greater for YFP pigs compared with NC pigs. However, results also indicated that the influence of YFP on the piglet intestinal microenvironment might differ when given in feed or water during ETEC challenge. In conclusion, pigs receiving YFP showed a better appetite in the presence of ETEC, which, together with the greater ileal digesta bacteria richness and diversity and decreased ETEC adhering to the mucosa and reduced colonic ammonia, indicates a healthier GIT environment. 

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