Interest in lignocellulose breakdown in the gut has centered largely on the rumen, mainly be­cause ofthe nutritional and economic importance ofruminal degradation, but also because of its relative accessibility as a gut microbial ecosystem. The large intestine of herbivores such as horses, however, harbor anaerobic communities of similar diversity and complexity (82) and include anaerobic bacteria, fungi, and protozoa. The major cellulolytic bacteria again appear to species of Ruminococcus and Fibrobacter (83). There is no indication that omnivores, including man, carry cellulolytic eukaryotes, but cellulolytic bacteria related to Ruminococcus spp. have been isolated (84). In addition, there is evidence that unknown Clostridium-related bacteria from human faecal samples can attach closely to wheat bran (85).

12.3 Conclusions

It is increasingly clear that the pool of DNA sequences coding for polysaccharide-degrading activities within the rumen microbial community has been subject to horizontal genetic exchange, including at some stage exchange between prokaryotic and eukaryotic microor­ganisms. The protein structures to which these catalytic and binding domains contribute, however, show an extraordinary diversity among rumen microorganisms. It seems safe to assume that the macromolecular organization of enzymes, adhesion mechanisms, and trans­port systems plays a crucial role in determining the ecological niche occupied by a given species. The details of this relationship, however, remain to be clarified.

Complex multidomain organization in polysaccharidases seems to be a feature of the primary cellulolytic species of anaerobic bacteria and fungi found in the rumen. At least in R. flavefaciens and cellulolytic rumen anaerobic fungi, a significant proportion of these polypeptides area organized into multienzyme cellulosome-type complexes that are an­chored to the cell surface. While it is assumed that these complexes play a key role in cellu­lose and plant cell wall decomposition, as in cellulolytic Clostridia (86), functional evidence is still lacking. Furthermore, the mechanisms by which R. albus and F. succinogenes, two cellulolytic rumen bacteria that produce complex multidomain enzymes but have not been reported to display cellulosome organization, anchor their enzymes to the cell surface and achieve efficient plant cell wall breakdown remain unclear. Cellulolytic protozoa apparently achieve plant cell wall breakdown despite producing soluble enzymes of relatively simple structure, from the limited information currently available. These organisms may present a special case, however, since digestion is assumed to occur within food vacuoles that also contain ingested cellulolytic bacteria.

Analysis of completed genomes for representative cellulolytic microorganisms should soon provide crucial insights into their degradative enzyme systems. This information will need to be illuminated by functional studies, however, with a continuing requirement for gene transfer methodologies that can be applied to rumen microorganisms.

Acknowledgment

The Rowett Research Institute receives funding from the Scottish Executive Environment and Rural Affairs Department.