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Summary: Two strains of Ruminococcus flavefaciens, an important cellulose-decomposing bacterium, were isolated, one from the rumen of a sheep, the other from the rumen of a cow. Pure cultures were obtained by using the dilution method in agar media containing a strip of filter-paper. These strictly anaerobic, Gram-positive streptococci attack cellulose and cellobiose, but not starch, maltose, lactose or xylose. Only one strain could use glucose. Colonies on cellulose media were characterized by the formation of a yellow pigment; in cellobiose media the colonies were white. Growth on cellulose was favoured by addition of Clostridium sporogencs or a certain amount of sterilized medium in which Cl. sporogenes had previously grown.
Estimations of the end-products of fermentation of cellulose and cellobiose showed that at least 25% of the carbon could be recovered as succinic acid, c. 23% as acetic acid and c. 10% as formic acid; ethanol was absent and gas formation very limited. A description of the genus and the species is given.