Competition for cellobiose among three predominant ruminal cellulolytic bacteria under substrate-excess and substrate-limited conditions.

The ruminal cellulolytic bacteria Ruminococcus flavefaciens FD-1 and Fibrobacter succinogenes S85 coexisted in substrate-excess coculture with about equal population size, but R. flavefaciens outcompeted F. succinogenes for cellobiose in the substrate-limited cocultures whether the two strains were coinoculated or a steady-state culture of F. succinogenes was challenged by R. flavefaciens. This outcome of competition between these two strains is due to a classical pure and simple competition mechanism based on affinity for cellobiose. Although the population size of F. succinogenes was much higher (> 70%) than that of another cellulolytic species, Ruminococcus albus 7 in substrate-excess coculture, F. succinogenes was replaced by a population of R. albus in the substrate-limited coculture in both coinoculation and challenge experiments. R albus outcompeted F. succinogenes, apparently due to selection in the chemostat of a population of R. albus with a higher affinity for cellobiose. R. albus also outcompeted R. flavefaciens under substrate-limited conditions.