Feeding a Saccharomyces cerevisiae fermentation product (Olimond BB) does not alter the fecal microbiota of racehorses. Olimond BB

Saccharomyces cerevisiae fermentation products (SCFP) are feed supplements and are widely used in animal nutrition to promote health. 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 study whether a previously observed immunomodulatory effect of SCFP feeding in racehorses was due to a feeding-induced altered gut microbiome, we analyzed the fecal microbiota. Horses were fed either the SCFP (Olimond BB) or a placebo product for 10 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 an 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 main effects of SCFP are due to its postbiotic ingredients.