COMPETITION FOR CELLOBIOSE AMONG 3 PREDOMINANT RUMINAL CELLULOLYTIC BACTERIA UNDER SUBSTRATE-EXCESS AND SUBSTRATE-LIMITED CONDITIONS

The ruminal cellulolytic bacteria Ruminococcus flavefaciens FD-1 and F ibrobacter succinogenes S85 coexisted in substrate-excess coculture wi th about equal population size, but R. flavefaciens outcompeted F. suc cinogenes for cellobiose in the substrate-limited cocultures whether t he two strains were coinoculated or a steady-state culture of F. succi nogenes was challenged by R. flavefaciens. This outcome of competition between these two strains is due to a classical pure and simple compe tition mechanism based on affinity for cellobiose. Although the popula tion size of F. succinogenes was much higher (>70%) than that of anoth er cellulolytic species, Ruminococcus albus 7 in substrate-excess cocu lture, F. succinogenes was replaced by a population of R. albus in the substrate-limited coculture in both coinoculation and challenge exper iments. R. albus outcompeted F. succinogenes, apparently due to select ion in the chemostat of a population of R. albus with a higher affinit y for cellobiose. R. albus also outcompeted R. flavefaciens under subs trate-limited conditions.