Members of the CFB phylum, which includes Prevotella and Bacteroides, account for a very significant proportion (>30%) of total rumen bacteria (43, 50-51). Many of these species appear to be unique to the rumen (52). None of the available isolates are known to be cellulolytic, but a number of species, notably P. bryantii and P. ruminicola, possess carboxymethylcellulases, hemicellulases, and pectinases (53). This suggests a role in utiliz­ing products of plant cell wall breakdown released by primary degraders, as suggested by coculture studies that demonstrate cross-feeding of oligosaccharides (2, 3, 54).

Gene clusters concerned with polysaccharide utilization have been studied in P. bryantii B14. One six-gene operon encodes two glucanases and a mannanase (55). The main role of at least one of the P. bryantii glucanases maybe in degrading p (1,3-1,4) glucans (56, 57). Another operon encodes a family 43 xylosidase/exoxylanase and a family 10 endoglucanase (58) and shows homology with an operon from Bacteroides ovatus that is essential for xylan utilization (59). Expression of this gene cluster in P. bryantii is upregulated in response to substrate availability (60) with xylo-oligosaccharides providing the induction signal (61). Most of the xylanase activity found in P. bryantii cells is released only upon cell disruption, suggesting that it is located in the periplasm or membranes (62). The situation resembles that for starch-degrading enzymes in the human gut bacterium, Bacteroides theotaiotaomicron, where limited hydrolysis is thought to occur at the outer membrane, followed by extensive hydrolysis in the periplasm (63). An unusual family 10 xylanase encoded by an unlinked gene XynC (64) that has a preference for large xylo-oligosaccharides is a possible candidate for the limited extracellular xylanase activity in P. bryantii Bi4.

Genome sequences are available for some non-rumen representatives of the CFB divi­sion. The human colonic strain, Bacteroides thetaiotaomicron 5482, has a large genome (6.26 Mb) that exhibits considerable redundancy with respect to polysaccharide-degrading en­zymes (65). This species is not a primary degrader of plant cell walls, and does not possess cellulases or xylanases belonging to the two major GH families 10 or 11, but possesses multiple xylosidase and glucosidase genes. Genome sequencing of rumen Prevotella species is likely to reveal greater complexity of xylanases and related enzymes than is known at present.