succinogenes than by co-culture with clade II isolates. Quantitative PCR analysis showed that bacterial abundance in the rumen was higher for clade I than for clade II. These results suggest that S. ruminantium, in particular click here the major clade I, is involved in rumen fiber digestion by cooperating with F. succinogenes. Fiber fermentation in the rumen is of critical importance for efficient production in ruminant animals. The ability to digest plant fiber has been ascribed to complex rumen microbiota consisting of bacteria, archaea, fungi, and protozoa that are closely interrelated. It is

generally accepted that ruminal fibrolysis is primarily because of bacterial activity, in particular to the activity of three predominant species: Fibrobacter succinogenes, Ruminococcus

albus, and Ruminococcus flavefaciens (Forsberg et al., 1997). However, not only these fibrolytic species, but also nonfibrolytic species are important for fiber degradation in the rumen, because nonfibrolytic bacteria can activate fibrolytic bacteria through an interaction termed ‘cross-feeding’ (Wolin et al., 1997). Nonfibrolytic Treponema bryantii (Kudo et al., 1987) and Prevotella ruminicola (Fondevila & Dehority, 1996) have been reported to synergize with fibrolytic bacteria to improve fiber digestion. Interspecies hydrogen transfer and removal and/or exchange of metabolites are factors that are considered to contribute to such synergism (Wolin et al., 1997). Selenomonas ruminantium is another nonfibrolytic bacterium Lumacaftor that may interact with fibrolytic bacteria, because this species is detected with high frequency as a major member of the fiber-attaching bacterial population (Koike et al., 2003b). Indeed, S. ruminantium improves fiber digestion when co-cultured with R. flavefaciens by the conversion of succinate, a metabolite of R. flavefaciens, enough into propionate (Sawanon & Kobayashi, 2006). A similar relationship was speculated for the combination of S. ruminantium and F. succinogenes by Scheifinger & Wolin (1973), who found that this combination of bacteria resulted in a synergistic increase in propionate

production. However, the synergistic improvement in fiber digestion was not quantified. Evaluation of this synergy is essential for the maximization of rumen fiber digestion because F. succinogenes is considered to be the most important fibrolytic species for rumen fiber digestion (Kobayashi et al., 2008). Recent molecular studies on rumen bacteria have revealed that some of the nonfibrolytic bacterial species are diverse in terms of their phylogeny and functions (Bekele et al., 2010, 2011). Selenomonas ruminantium also appears to be functionally diverse, because S. ruminantium HD4 possesses CMCase, whereas other S. ruminantium strains do not. The strain HD4 also possesses xylanolytic activity, even though it is weak (Hespell et al., 1987). Pristas et al.