Metabolism of D-glucose by Bacteroides ruminicola subsp. brevis, strain B,4, has been examined. Growth yield studies gave molar growth yields, corrected for storage polysaccharide, of approximately 66 g (dry weight)/mol of glucose fermented. The storage polysaccharide amounted to about 14% of the total dry weight, or 55% of the total cellular carbohydrate, at full growth. After correcting glucose utilization for incorporation into cellular carbohydrate, measurement of product formation showed that 1.1 succinate, 0.8 acetate, and 0.35 formate are produced and 0.5 CO2 net is taken up during the fermentation of 1 glucose under the conditions used. The implication of these results with respect to adenosine 5'-triphosphate (ATP) molar growth yield calculations is discussed. If substratelevel phosphorylation reactions alone are responsible for ATP generation, then the ATP molar growth yield must be about 23 g (dry weight)/mol of ATP. Alternatively, if anaerobic electron transfer-linked phosphorylation also occurs, the ATP molar growth yield will be lower. Bacteroides ruminicola is one of the more numerically important bacteria found in the rumen, representing 6 to 19% of the total culturable carbohydrate fermenters present (8). It is not immediately obvious why B. ruminicola can compete so successfully in this anaerobic environment, which contains so many other carbohydrate-utilizing, organic acid-producing bacteria. Part of the reason for its quantitative predominance may be the result of its great versatility towards possible carbohydrate and nitrogen sources utilized (7, 8). However, it is also likely that the species has a highly efficient energy metabolism and can multiply rapidly. This study set out to examine growth yields of B. ruminicola, as an index of its energy metabolism, to discover whether the yields are "normal" or "anomalous" (19) and can be predicted from the products of fermentation. Careful attention has been paid to correction for storage polysaccharide and carbon recovery in products, which can both lead to erroneous results if ignored. B. ruminicola has a b-type cytochrome, which may be involved in fumarate reduction by reduced nicotinamide adenine dinucleotide (40); such electron transport systems in other bacteria have sometimes been postulated to be coupled to anaerobic electron transport phosphorylation reactions (3, 16, 28). If such a system is operative in B. ruminicola, then adenosine 5'-triphosphate (ATP) yields will be higher than can be accounted for by substrate-level phosphorylation reactions. Caldwell (D. R. Caldwell, Diss. Abstr. Int. B. 31:1419) has previously suggested that ATP yields in B. ruminicola strain GA33 are between 4.5 and 7.8 ATP/ mol of glucose, based on measurements of cellular protein. MATERIALS AND METHODS Bacteria. B. ruminicola subsp. brevis, strain B,4 (7, 8), was used in the present work and was kindly supplied by M. P. Bryant of the University of Illinois. This strain was used in preference to the type strain, strain GA33, because strain B,4 clumps rather less ...