Fibrobacter succinogenes and F. intestinalis are specialized cellulolytic bacteria that belong to a divergent phylum of Gram-negative anaerobes (42). Significant Fibrobacter populations can be detected by molecular probing (8), although they are poorly represented in ampli­fied 16S clone libraries (43). Genome sequencing of F. succinogenes S85 has identified 113 genes that have likely roles in plant cell wall degradation, including 40 cellulases and 29 xy — lanases (44). As in Gram-positive cellulolytic anaerobes such as the ruminococci, complex organization involving multiple catalytic domain and substrate-binding modules is a feature of F. succinogenes enzymes (1, 45). Dockerin-like domains have not been detected, however, and the organization of cellulolytic enzymes on the cell surface remains unclear. Some en­zymes apparently share a basic C-terminal region, but it has not been established whether this is involved in cell surface attachment. Gene complement and cellulolytic activity are apparently well conserved among strains related to F. succinogenes S85, but with evidence of significant sequence divergence in F. intestinalis and some F. succinogenes-related strains (44, 46, 47).

F. succinogenes apparently lack xylose isomerase activity (48) and fail to utilize the break­down products of xylan, despite possessing an array of xylanases. R. flavefaciens strains vary in their ability to utilize xylo-oligosaccharides, and appear to vary in respect of their xylose isomerase genes (49). For these bacteria, xylanase activity, therefore, appears to be required primarily for degrading the matrix polysaccharides thus facilitating access to the glucan components of the plant cell wall.