Microbiology (2001), 147, 21–30 Printed in Great Britain Competition among three predominant ruminal cellulolytic bacteria in the absence or presence of non-cellulolytic bacteria Junqin Chen 1 and Paul J. Weimer 1,2 Author for correspondence : Paul J. Weimer. Tel :1 608 264 5408. Fax :1 608 264 5147. e-mail : pjweimerfacstaff.wisc.edu 1 Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA 2 United States Department of Agriculture, Agricultural Research Service, US Dairy Forage Research Center, USDA-ARS, 1925 Linden Drive West, Madison, WI 53706, USA Competition among three species of ruminal cellulolytic bacteria – Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD-1 and Ruminococcus albus 7– was studied in the presence or absence of the non-cellulolytic ruminal bacteria Selenomonas ruminantium or Streptococcus bovis. Co-cultures were grown under either batch or continuous conditions and populations were estimated using species-specific oligonucleotide probes to 16S rRNA. The three cellulolytic species co-existed in cellobiose batch co-culture, but inclusion of either Sel. ruminantium or Str. bovis yielded nearly a monoculture of the non- cellulolytic competitor. In cellobiose chemostats, R. albus completely dominated the triculture, but R. flavefaciens became predominant over F. succinogenes and R. albus when Sel. ruminantium was co-inoculated into the chemostats. Similar effects on competition were observed in the presence of Str. bovis at a lower (0021 h N1 ), but not at a higher (0045 h N1 ) dilution rate. In cellulose batch co-cultures, R. albus was more abundant than both F. succinogenes and R. flavefaciens, regardless of the presence of the non- cellulolytic species. Co-existence among the three cellulolytic species was observed in almost all cellulose chemostats, but Sel. ruminantium altered the relative proportions of the cellulolytic species. R. albus and R. flavefaciens were found to produce inhibitors that suppressed growth of R. flavefaciens and F. succinogenes, respectively. These data indicate that interactions among cellulolytic bacteria, while complex, can be modified further by non-cellulolytic species. Keywords : cellobiose, cellulose, competition, Fibrobacter succinogenes, Ruminococcus INTRODUCTION Numerous cellulolytic microbial species have been identified in the rumen, but it is generally agreed that ruminal cellulolysis is carried out primarily by three species of bacteria : Fibrobacter succinogenes, Ruminococcus flavefaciens and Ruminococcus albus (Hungate, 1966 ; Dehority, 1993). Because these species are nutritional specialists that differ in fermentation end products, and because cellulose is a major component of the diets of forage-fed ruminants, the relative popu- lations of these three species can potentially impact on the ratios of volatile fatty acids available to the animal, an important determinant of animal performance. Several studies have examined competition among these cellulolytic species. Odenyo et al. (1994a, b) used oligo- nucleotide probes to species-specific segments of 16S rRNA to quantify specific populations in binary (two- membered) and ternary (three-membered) batch cultures grown on cellobiose, cellulose or alkaline hydrogen peroxide-treated wheat straw. Their data indicated that R. albus 8 generally out-competed R. flavefaciens FD-1, due to production by the former of a bacteriocin-like substance. Fondevila & Dehority (1996) presented evidence that cellulose digestion was reduced when strains of F. succinogenes A3c and R. flavefaciens B34b were grown together in batch culture and suggested that an inhibitor was produced by one of the two species. Mosoni et al. (1997) reported that R. flavefaciens FD-1 became detached from cellulose in the presence of R. albus 20. Within a permissive range of ruminal pH, digestion of cellulose is considered to be a first-order process that is limited by the available surface area of cellulose rather than by the hydrolytic capabilities of the cellulolytic 0002-4395  2001 SGM 21