, 2003b; Shinkai, unpublished results). The results of the present study, which employed

both mono- and co-culture studies, strongly support this possibility. None of the S. ruminantium isolates could independently digest fiber as previously reported (Kingsley & Hoeniger, 1973; Scheifinger & Wolin, 1973), whereas the addition of S. ruminantium to a culture of F. succinogenes significantly improved fiber digestion with a concomitant increase in propionate production. This synergy find more could be caused by cross-feeding between the two species. Thus, F. succinogenes degrades cellulose to produce succinate and cello-oligosaccharides, while S. ruminantium decarboxylates succinate to propionate (Scheifinger & Wolin, 1973; Strobel C646 solubility dmso & Russell, 1991) and utilizes cello-oligosaccharides, some of which are known to function as feedback inhibitors of F. succinogenes cellulase (Huang & Forsberg, 1990; Maglione et al., 1997). More importantly, the extent of this synergy between F. succinogenes and S. ruminantium might depend on the phylotype of S. ruminantium, because clade I isolates were found to be more potent than clade II isolates in terms of increasing fiber digestion and propionate production. This result

could be explained by the superior ability of clade I isolates in succinate conversion, cello-oligosaccharide consumption or special niche formation, or by other unknown factors. It is therefore a priority to define the metabolic and ecologic advantages of clade I isolates that lead to their enhanced synergy with F. succinogenes compared with clade II isolates. This synergy between F. succinogenes and S. ruminantium aminophylline for fiber digestion only occurred on orchardgrass hay and rice straw but not on alfalfa. Although the reason for this difference is not apparent, it may depend on structural and chemical differences between fiber sources such as grasses and legumes (Akin et al., 1993). Indeed, S. ruminantium has often been found in bacterial 16S rRNA gene clones retrieved from ruminally incubated orchardgrass hay but has never found in clones

retrieved from ruminally incubated alfalfa hay (Koike et al., 2003b). However, Fibrobacter and Treponema species may synergize for the digestion of alfalfa as described by Stanton & Canale-Parola (1980), because ruminally incubated alfalfa yields several clones that show high similarity with Treponema (Koike et al., 2003b). Overall, clades I may be better symbionts for F. succinogenes in terms of grass fiber digestion. The S137 isolate (clade I) showed the highest synergy with F. succinogenes, which is in good agreement with a previous report regarding combinations of S. ruminantium and R. flavefaciens (Sawanon & Kobayashi, 2006). Active decarboxylation of succinate to produce propionate, which was previously demonstrated for the combination of S. ruminantium and R.