Yeast Products Mediated Ruminal Subenvironmental Microbiota, and Abnormal Metabolites and Digestive Enzymes Regulated Rumen Fermentation Function in Sheep

SIMPLE SUMMARY: Increasing the proportion of concentrate in the diet is a common strategy to improve the performance of ruminants. However, this may lead to abnormal rumen fermentation, which is a threat to animal health. As a green feed additive, yeast product (YP) is often used to regulate rumen fermentation and maintain animal health, but its mechanism of action on ruminants is unclear. This study was conducted to investigate the effects of YP (yeast culture + yeast live cells) on rumen fermentation parameters, abnormal metabolites and microbiota in sheep. Our findings suggest that YP can improve rumen fermentation parameters, change the activity of rumen digestive enzymes and reduce the concentration of abnormal metabolites. In addition, we found that YP significantly changed the relative abundance of microorganisms attached to rumen feed particles and affected microbial function. This study provides new insights into the regulation mechanism of feed additives on rumen fermentation and improves our understanding of YP on rumen fermentation regulation. ABSTRACT: Yeast products (YP) are commonly used as rumen regulators, but their mechanisms of action are still unclear. Based on our previous studies, we questioned whether yeast products would have an impact on rumen solid-associated (SA) and liquid-associated (LA) microorganisms and alter rumen fermentation patterns. Thirty 3-month-old male sheep weighing 19.27 ± 0.45 kg were selected and randomized into three groups for 60 days: (1) basal diet group (CON group), (2) basal diet add 20 g YP per day (low YP, LYP group) and (3) basal diet add 40 g YP per day (high YP, HYP group). The results demonstrated that the addition of YP increased rumen cellulase activity, butyrate and total volatile fatty acid (TVFA) concentrations (p < 0.05), while it decreased rumen amylase activity and abnormal metabolites, such as lactate, lipopolysaccharides (LPS) and histamine (HIS) (p < 0.05). Metagenomic analysis of rumen microorganisms in three groups revealed that YP mainly influenced the microbial profiles of the SA system. YP increased the relative abundance of R. flavefaciens and decreased methanogens in the SA system (p < 0.05). With the addition of YP, the abundance of only a few lactate-producing bacteria increased in the SA system, including Streptococcus and Lactobacillus (p < 0.05). However, almost all lactate-utilizing bacteria increased in the LA system, including Megasphaera, Selenomonas, Fusobacterium and Veillonella (p < 0.05). In addition, YP increased the abundance of certain GHs family members, including GH43 and GH98 (p < 0.05), but decreased the abundance of some KEGG metabolic pathways involved in starch and sucrose metabolism, biosynthesis of antibiotics and purine metabolism, among others. In conclusion, the addition of YP to high-concentrate diets can change the abundance of major functional microbiota in the rumen, especially in the solid fraction, which in turn affects rumen fermentation patterns and improves rumen digestibility.