53. A Soluble Yeast Cell Wall Preparation Alters Fermentation Products Without Affecting Microbial Diversity in the Feline Gut.

Abstract Yeast cell walls (YCW) are known to modulate microbiota composition and metabolic activity in several animal species [1], although remain little studied in cats. This study evaluated the effects of increasing dietary inclusion levels of a soluble YCW preparation from Saccharomyces cerevisiae on total tract apparent digestibility (TTAD) of nutrients, fecal fermentation products, immunoglobulin A (IgA), and microbiota in adult cats. Thirty-two healthy cats were assigned to four diets containing 0%, 0.2%, 0.4%, or 0.8% YCW (as-fed) in a randomized block design (n = 8; approval no. 3074/22). After a 15-day washout on control diet, animals were assigned to the experimental diets for 30 days. Fecal IgA was measured at baseline and at the end of the experimental period. TTAD were determined after a 15-day adaptation period. At day 30, fecal characteristics, pH, fermentation products (short- (SCFA) and branched-chain fatty acids (BCFA), lactate, ammonia), and microbiota (16S, and qPCR including Dysbiosis Index-DI) were assessed. Data were analyzed by ANOVA and polynomial contrasts (P < 0.05). For fecal IgA, baseline values were included as a covariate. 16S Alpha diversity was compared by Kruskal-Wallis, beta diversity by ANOSIM, and differential abundance using ANCOM-BC. The TTAD of most nutrients was unaffected, except for dietary fiber, which was higher in the 0.8% diet (P < 0.05). Fecal moisture and score were unchanged, while pH increased quadratically with YCW inclusion (P < 0.05). Acetate, propionate, total SCFA, valerate, isovalerate, lactate, and ammonia were not affected, whereas butyrate, isobutyrate, and BCFA increased linearly with YCW inclusion (P < 0.05). Fecal IgA did not differ (P > 0.05). Microbiota analysis showed no differences in alpha or beta diversity or DI, with all treatments presenting mean DI < 0, indicating no dysbiosis [2]. However, ANCOM BC identified family-level shifts: Ruminococcaceae and Oscillospiraceae decreased at 0.2%, while Ethanoligenenaceae increased at 0.4%. These taxa belong to Firmicutes/Clostridia, a group of anaerobic fermentative bacteria involved in carbohydrate metabolism and SCFA production. The reduction of Ruminococcaceae and Oscillospiraceae may reflect a transient microbial adaptation to yeast-derived fermentable substrates rather than a detrimental shift, particularly considering the shorter experimental period and the absence of this effect at higher inclusions [3]. In contrast, Ethanoligenenaceae remains poorly characterized in feline gut microbiota, limiting functional interpretation. Importantly, these taxonomic differences represent relative abundance changes that do not necessarily reflect microbial metabolic activity. Overall, the soluble YCW preparation modulated fermentation products without altering overall microbial diversity, suggesting functional shifts in microbial metabolism independent of major taxonomic restructuring. The linear increase in fecal butyrate further indicates potential benefits for gut health in cats. References: [1] Kyoung et al. (2023) Poultry Sci, 102(6):102660; [2] Sung et al. (2022) J Feline Med Surg, 24(6):e1-e12; [3] Liu X et al. (2025) Food Sci Nutri, 13:e71007