The presence of yeast cells could stimulate hydrogen utilization of acetogens and enhance acetogenesis. To understand the roles of acetogens in rumen fermentation, an in vitro rumen fermentation experiment was conducted with addition of acetogen strain （TWA4） and/or Saccharomyces cerevisiae fermentation product （XP）. A 2×2 factorial design with two levels of TWA4 （0 or 2×10^7 cells/ml） and XP （0 or 2 g/L） was performed. Volatile fatty acids （VFAs） were increased （P〈0.05） in XP and TWA4XP, while methane was increased only in TWA4XP （P〈0.05）. The increase rate of microorganisms with formyltetrahydrofolate synthetase, especially acetogens, was higher than that of methanogens under all treatments. Lachnospiraceae was predominant in all acetogen communities, but without close acetyI-CoA synthase （ACS） amino acid sequences from cultured isolates. Low-Acetitomaculum ruminis-like ACS was predominant in all acetogen communities, while four unique phylotypes in XP treatment were all amino acid identified Iow-Eubacterium limosum-like acetogens. It differs to XP treatment that more Iow-A. ruminis-like and less Iow- E. limosum-like sequences were identified in ＂I＇WA4 and TWA4XP treatments. Enhancing acetogenesis by supple- mentation with an acetogen strain and/or yeast cells may be an approach to mitigate methane, by targeting proper acetogens such as uncultured Iow-E. limosum-like acetogens.
