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

Three predominant ruminal cellulolytic bacteria (Fibrobacter succinoge nes S85, Ruminococcus flavefaciens FD-1, and Ruminococcus albus 7) wer e grown in different binary combinations to determine the outcome of c ompetition in either cellulose-excess batch culture or in cellulose-li mited continuous culture. Relative populations of each species were es timated by using signature membrane-associated fatty acids and/or 16S rRNA-targeted oligonucleotide probes. Both F. succinogenes and R. flav efaciens coexisted in cellulose-excess batch culture with similar popu lation sizes (58 and 42%, respectively; standard error, 12%). By contr ast, under cellulose limitation R. flavefaciens predominated (>96% of total cell mass) in coculture with F. succinogenes, regardless of whet her the two strains were inoculated simultaneously or whether R. flave faciens was inoculated into an established culture of F. succinogenes. The predominance of R. flavefaciens over F. succinogenes under cellul ose limitation is in accord with the former's more rapid adherence to cellulose and its higher affinity for cellodextrin products of cellulo se hydrolysis. In batch cocultures of F. succinogenes and R. albus, th e populations of the two species were similar. However, under cellulos e limitation, F. succinogenes was the predominant strain (similar to 8 0% of cell mass) in cultures simultaneously coinoculated with R. albus . The results from batch cocultures of R. flavefaciens and R.; albus w ere not consistent within or among trials: some experiments yielded mo nocultures of R. albus (suggesting production of an inhibitory agent b y R. albus), while others contained substantial populations of both sp ecies. Under cellulose limitation, R. flavefaciens predominated over X . albus (85 and 15%, respectively), as would be expected by the former 's greater adherence to cellulose. The retention of R. albus in the ce llulose-limited coculture may result from a combination of its ability to utilize glucose (which is not utilizable by R. flavefaciens), its demonstrated ability to adapt under selective pressure in the chemosta t to utilization of lower concentrations of cellobiose, a major produc t of cellulose hydrolysis, and its possible production of an inhibitor y agent.