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S elenomonas ruminantium strains were isolated from sheep rumen, and their significance for fiber digestion was evaluated. Based on the phylogenetic classification, two clades of S . ruminantium (clades I and II) were proposed. Clade II is newly found, as it comprised only new isolates that were phylogenetically distant from the type strain, while all of the known isolates were grouped in the major clade I. More than half of clade I isolates displayed CMCase activity with no relation to the degree of bacterial adherence to fibers. Although none of the isolates digested fiber in monoculture, they stimulated fiber digestion when co-cultured with F ibrobacter succinogenes, and there was an enhancement of propionate production. The extent of such synergy depended on the clade, with higher digestion observed by co-culture of clade I isolates with F . succinogenes than by co-culture with clade II isolates. Quantitative PCR analysis showed that bacterial abundance in the rumen was higher for clade I than for clade II. These results suggest that S . ruminantium, in particular the major clade I, is involved in rumen fiber digestion by cooperating with F . succinogenes. [ABSTRACT FROM AUTHOR]