CELLODEXTRIN EFFLUX BY THE CELLULOLYTIC RUMINAL BACTERIUM FIBROBACTER-SUCCINOGENES AND ITS POTENTIAL ROLE IN THE GROWTH OF NONADHERENT BACTERIA

When glucose or cellobiose was provided as an energy source for Fibrobacter succinogenes, there was a transient accumulation (as much as 0.4 mM hexose equivalent) of cellobiose or cellotriose, respectively, in the growth medium. Nongrowing cell suspensions converted cellobiose to cellotriose and longer-chain cellodextrins, and in this case the total cellodextrin concentration was as much as 20 mM (hexose equivalent). Because cell extracts of glucose- or cellobiose-grown cells cleaved cellobiose and cellotriose by phosphate-dependent reactions and glucose 1-phosphate was an end product, it appeared that cellodextrins were being produced by a reversible phosphorylase reaction. This conclusion was supported by the observation that the ratio of cellodextrins to cellodextrins with one greater hexose [n/(n + 1)] was approximately 4, a value similar to the equilibrium constant (K-eq) of cellobiose phosphorylase (J. K. Alexander, J. Bacteriol, 81:903-910, 1961). When F. succinogenes was grown in a cellobiose-limited chemostat, cellobiose and cellotriose could both be detected, and the ratio of cellotriose to cellobiose was approximately 1 to 4. On the basis of these results, cellodextrin production is an equilibrium (mass action) function and not just an artifact of energy-rich cultural conditions. Cellodextrins could not be detected in low-dilution-rate, cellulose-limited continuous cultures, but these cultures' had a large number of nonadherent cells. Because the nonadherent cells had a large reserve of polysaccharide and were observed at all stages of cell division, it appeared that they were utilizing cellodextrins as an energy source for growth. The noncellulolytic bacterium Streptococcus Bovis persisted in batch culture with F. succinogenes even though cellulose was the only energy source, and the ratio of S. bovis to F. succinogenes was approximately 1 to 4. The carbohydrate metabolism off. succinogenes seems to reflect a compromise between the energetic advantage of a phosphorylase reaction and the potential loss of carbon and energy as extracellular cellodextrins.