Ruminal Yeast Strain with Probiotic Potential: Isolation and Characterization and Its Effect on Rumen Fermentation In Vitro

The objective of this study is to isolate, identify, and describe rumen yeast strains and assess their probiotic potentials and effects on ruminal fermentation in vitro. Yeasts were isolated from ruminal fluids, yielding 59 strains from nine distinct species. A number of tests were conducted to assess their anaerobic traits, growth rate, acid tolerance, and lactate utilization ability, and a second screening in fresh ruminal fluid to evaluate in vitro pH and acid accumulation was conducted. The probiotic yeast <i>Candida rugosa</i> (NJ-5) was selected for in vitro culture studies on rumen fermentation. Finally, <i>Candida rugosa</i> (NJ-5) with good probiotic characteristics was chosen to investigate its effects on ruminal fermentation in vitro. The batch culture technique was used to explore the effects of <i>Candida rugosa</i> (NJ-5) yeast culture on rumen fermentation parameters. By altering the fermentation substrate to a concentrate-to-roughage ratio of 70:30, which simulated a high-concentration diet. The CON, LYC, MYC, and HYC groups were supplemented with 0%, 1%, 2%, and 5% <i>Candida rugosa</i> (NJ-5) yeast culture (dry matter basis), respectively. The pH value and volatile fatty acid (VFA) contents were determined at 6, 12, and 24 h after fermentation. The results showed that adding <i>Candida rugosa</i> (NJ-5) yeast culture successfully modulated in vitro rumen fermentation. Compared to the CON group, HYC had a significantly mitigated reduction in pH in fermentation, resulting in a significant increase in total VFAs and acetate levels (<i>p</i> < 0.05). Additionally, 16S rRNA sequencing revealed that <i>Candida rugosa</i> (NJ-5) yeast culture supplementation did not significantly alter ruminal bacterial alpha diversity (<i>p</i> > 0.05). At the phylum and genus taxonomic levels, <i>Candida rugosa</i> (NJ-5) yeast culture addition increased the relative abundance of several functionally important bacterial groups in the rumen microbial community. Compared to the CON group, the HYC group concurrently had an increased abundance of Desulfobacterota, <i>Christensenellaceae_R-7_group</i>, <i>F082</i>, and <i>Ruminococcus</i> (<i>p</i> < 0.05) but a significantly reduced abundance of Cyanobacteria, Bdellovibrionota, <i>Succinivibrionaceae_UCG-002</i>, <i>Enterobacter</i>, and <i>Succinivibrio</i> (<i>p</i> < 0.05). The in vitro fermentation experiment demonstrated that the optimal dry matter supplementation of <i>Candida rugosa</i> (NJ-5) into the basal diet was 5%, which could be effective for maintaining ruminal fermentation stability when ruminants were fed a high-concentrate diet. This study provides empirical support for the use of yeast as a nutritional supplement in ruminant livestock management, as well as a theoretical underpinning for further animal research.