A gram-negative, anaerobic, nonmotile, non-spore-forming, rod-shaped bacterium that fermented succinate quantitatively to propionate was isolated from a high dilution of rumen ingesta obtained from a dairy cow fed a production diet containing grass silage as the main roughage source. This organism did not grow on any of the following energy sources: 12 carbohydrates, pyruvate, lactate, 7 dicarboxylic acids, aspartate, citrate, and -aconitate. Both rumen fluid and yeast extract were necessary for good growth on succinate. The organism was negative for the following characteristics: production of propionate from threonine, protein digestion, sulfide production, nitrate reduction, catalase activity, and urease activity. There was no growth at 22°C and reduced growth at 45°C compared with growth at 39°C. The DNA base composition was 52 mol% G+C. The complete 16S rRNA sequence (EMBL accession number, X81137) was obtained, and the phylogenetic relationships of the organism were determined. The most closely related genera were the genera and . The name proposed for this bacterium is gen. nov., sp. nov.; the type strain is strain SE10 (= DSM 9236). Additional isolation attempts revealed that is a common inhabitant of the rumina of cows that are fed production diets and of cows on pasture.


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