Feeding a Saccharomyces cerevisiae Fermentation Product (Olimond BB) Does Not Alter the Fecal Microbiota of Thoroughbred Racehorses

Simple Summary Saccharomyces cerevisiae fermentation products (SCFP) are feed supplements and are widely used in animal nutrition to promote health. The biological effects of SCFP are based on prebiotic mechanisms that directly influence the microbial community of the gut microbiome or postbiotic factors that directly interact with host cells. To show whether the immunomodulatory effects of SCFP feeding are due to an altered composition of gut microbiota, we analyzed the fecal microbiota of racehorses. Horses were fed either the SCFP (Olimond BB) or a placebo product for six weeks, and fecal samples were collected for 16S rRNA gene sequencing. During this period, SCFP feeding only subtly affected the fecal microbiota in bacterial composition and diversity. SCFP and placebo horses differed significantly in the fecal bacterial diversity directly after intramuscular influenza vaccination. Altogether, the findings argue against a strong prebiotic effect of SCFP in racehorses. In contrast, the modulation of vaccine- and host-induced alterations of the microbiome suggests that the main effects of SCFP are due to contained or induced postbiotic components. Feed supplements such as Saccharomyces cerevisiae fermentation products (SCFP) alter immune responses in horses. The purpose of this study was to analyze whether a prebiotic activity of the SCFP alters the gut microbiome in horses. Racehorses were fed either SCFP (Olimond BB, OLI, n = 6) or placebo pellets (PLA, n = 5) for 43 days. Fecal microbiota analysis was performed using 16S rRNA gene sequencing. The numbers and function of circulating immune cell subpopulations were analyzed by flow cytometry. SCFP supplementation resulted in non-consistent differences in fecal microbiota between the PLA and OLI during the feeding period. Rather, the individual animal had the highest impact on fecal microbiota composition. OLI and PLA horses displayed the same changes in numbers of blood leukocyte subpopulations over time. One day after a booster vaccination against equine influenza during the feeding period, the alpha diversity of fecal microbiota of PLA horses was significantly higher compared to OLI horses. This suggests that SCFP feeding altered the vaccination-induced spectrum of released mediators, potentially affecting gut microbiota. The overall non-consistent findings argue against a strong prebiotic effect of Olimond BB on the microbiota in racehorses. Fecal microbiota differences between the groups were also noticed outside the feeding period and, hence, are most likely not caused by the SCFP additive.