Harry J. Flint

12.1 Introduction

Herbivorous mammals do not secrete digestive enzymes that are able to degrade the major structural polysaccharides of plant cell walls, but depend on the activities of symbiotic gut microorganisms to obtain energy from the plant material that makes up the bulk of their diet. Breakdown ofplant cell wall material is mediated by anaerobic microbial communities that develop in the large intestine (caecum and colon) in the case of hind-gut fermentors such as horses and rabbits, while in ruminants this breakdown occurs largely in the foregut, in the reticulo-rumen. In both cases, the short chain fatty acid products of fermentation are absorbed and used as energy sources by the animal, but the foregut location of the rumen also allows the animal to take advantage of microbial protein, by digesting the microbial cells that pass into the acidic stomach.

The rumen is the site of highly efficient breakdown of the wide variety of plant material that forms the diet of grazing animals, and harbors a complex consortium of anaerobic mi­croorganisms comprising bacteria, archaea, fungi, and protozoa (1). Plant polysaccharide­degrading enzymes are produced by a high proportion of these microorganisms, both eu­karyotes and prokaryotes. Many rumen microorganisms may, however, be considered as secondary utilizers that exist by cross-feeding, and are largely dependent on other organ­isms for primary attack upon the more recalcitrant plant structures (2-5). The role of their enzyme systems and transport machinery is to scavenge soluble polysaccharides and oligosaccharides as they are released by other microorganisms from plant material. The primary degraders of plant cell wall material on the other hand are assumed to be those that are capable of tight attachment and that possess the enzymatic machinery to access this complex, insoluble substrate. The number ofgenuinely cellulolytic species identified among the rumen microbiota is relatively small, and their populations may often be underestimated because of the difficulty of recovering them in the substrate. Interest has centered on these primary plant cell-wall-degrading species because of their key roles in initiating substrate breakdown.

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

12.2 Cellulolytic and hemicellulolytic bacteria from the rumen

Three rumen bacterial species, in particular, were recognized from early cultural studies to be actively cellulolytic. Two Ruminococcus species, R. flavefaciens and R. albus, are repre­sentatives of the Gram-positive Clostridial cluster IV, while Fibrobacter succinigenes belongs to a divergent group of Gram-negative bacteria. Recent molecular work has confirmed the importance of all three species in the rumen ecosystem (6-8) although there are indications from 16S rRNA diversity studies that hitherto uncultured cellulolytic bacteria remain to be recovered from the rumen (9).