Submitted to: Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2000
Publication Date: N/A
Citation: Interpretive Summary: Several species of bacteria in the rumen digest cellulose, the main component of forages. We examined the competition among three of these species after combining them in a fermentation vessel and feeding a culture medium with cellulose or cellobiose (a sugar derived from cellulose digestion) as energy sources. When cellobiose was fed, the species previously shown to be most abundant of the three in the rumen was also most abundant in the fermentor, and its dominance was most apparent when cellobiose concentrations were low, as in the rumen environment. When cellulose was fed, this species was again dominant when cellulose was abundant, but shared the reactor with the other two species when cellulose was limiting. When the three-membered culture was fed cellulose and challenged with the cellulose-nondigesting species, the challengers were able to remain in the culture at low populations, where they lived off of the small amounts of sugars left over from cellulose digestion. These cellulose-nondigesting species sometimes altered the competition among the cellulose-digesting species. The abundance of the dominant cellulose- digesting species was due in part to its production of a protein-containing factor that inhibited the growth of one of the less-abundant cellulose- digesting species. The results indicate that interactions among these species can be successfully modeled in a more defined laboratory environment, and that inhibitory agents produced by one species can affect the outcome of competition at both high and low concentrations of bacterial energy sources. This information will be useful to scientists constructing mathematical models of forage digestion by ruminants.
Technical Abstract: Competition among three ruminal bacteria -- Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD-1 and Ruminococcus albus 7 -- was studied in the presence or absence of the noncellulolytic ruminal bacteria Selenomonas ruminantium or Streptococcus bovis under both batch and continuous culture conditions, using species-specific oligonucleotide probes to 16S rRNA. The three cellulolytic species coexisted in cellobiose batch coculture, but inclusion of either S. ruminantium or S. bovis yielded nearly a monoculture of the noncellulolytic competitor. In cellobiose chemostats, R. albus completely dominated the triculture; R. flavefaciens became predominant over F. succinogenes and R. albus when S. ruminantium was coinoculated into the chemostats. Similar effects on the competition were observed in the presence of S. bovis at low (0.021 h**-1), but not high (0.045 h**-1), dilution rate. R. albus outcompeted both F. succinogenes and R. flavefaciens in cellulose batch cocultures in the absence or presence of S ruminantium or S.bovis. Coexistence among the three cellulolytic species was observed in almost all cellulose chemostats. Despite the reported positive interaction between F. succinogenes and S. ruminantium, addition of the latter organism to the triculture increased the relative population size of R. albus at the expense of F. succinogenes. Both R. albus and R. flavefaciens were found to produce an inhibitor that suppressed the growth of R. flavefaciens and F. succinogenes, respectively. The inhibitor from R. albus was shown to be sensitive to proteases, and its molecular weight was between 10 and 30 kDa.