The metabolism of succinic acid in the rumen of the sheep.

Succinic acid is a major end product of two cellulose-fermenting bacteria isolated in pure culture from the rumen of cattle and sheep. Ruminococcus flavefaciens, isolated by Sijpesteijn (1948, 1949, 1951), produced some 0-5 m-mole succinic acid per m-mole cellulose (calculated as glucose) fermented. Bacteroides succinogenes, isolated from the bovine rumen by Hungate (1950), was found by this author to produce 51-3 mg. succinic acid from 82 mg. cellulose. Since large amounts of cellulose are fermented each day in the rumen and in view of the importance of the two organisms mentioned above it is reasonable to expect to find a high concentration of succinate in the rumen; however, Marston (1948) found that only very small amounts of succinate were produced during the fermentation of cellulose in an artificial rumen. On the other hand, propionic acid is known to be formed in large quantities when cellulose is fermented in vitro (Elsden, 1945; Marston, 1948), and there is now evidence that propionic acid is formed by the decarboxylation of succinic acid (Johns, 1951a, b, c; Delwiche, 1948). A possible solution to this paradox is that succinic acid is formed in the rumen from cellulose and is rapidly decarboxylated to propionic acid. The present investigations deal with the production of succinic acid in the rumen during the digestion of a normal diet and with the fate of succinic acid added directly to the rumen. In addition, the metabolism of succinic acid by washed suspensions ofrumen bacteria has been investigated. This use of washed suspensions of rumen bacteria represents a new experimental approach to the study of the rumen, and some indications are given of the possible applications of this technique.