Paper-Lecture32 - Why Don't Ruminal Bacteria Digest...

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ABSTRACT The bacteria Fibrobacter succinogenes, Ruminococ- cus flauefaciens, and Ruminococcus albus generally are regarded as the predominant cellulolytic microbes in the rumen. Comparison of available data from the literature reveals that these bacteria are the most actively cellulolytic of all mesophilic organisms described to date from any habitat. In light of numer- ous proposals to improve microbial cellulose digestion in ruminants, it is instructive to examine the charac- teristics of these species that contribute to their su- perior cellulolytic capabilities and to identify the fac- tors that prevent them from digesting cellulose even more rapidly. As a group, these species have extreme nutritional specialization. They are able to utilize cellulose (or in some cases xylan) and its hydrolytic products as their nearly sole energy sources for growth. Moreover, each species apparently has evolved to similar maximum rates of cellulose diges- tion (first-order rate constants of 0.05 to 0.08 h-l). Active cellulose digestion involves adherence of cells to the fibers via a glycoprotein glycocalyx, which pro- tects cells from protozoal grazing and cellulolytic en- zymes from degradation by ruminal proteases while it retains-at least temporarily-the cellodextrin products for use by the cellulolytic bacteria. These properties result in different ecological roles for the adherent and nonadherent populations of each spe- cies, but overall provide an enormous selective advan- tage to these cellulolytic bacteria in the ruminal en- vironment. However, major constraints to cellulose digestion are caused by cell-wall structure of the plant (matrix interactions among wall biopolymers and low substrate surface area) and by limited penetration of the nonmotile cellulolytic microbes into the cell lumen. Because of these constraints and the highly adapted nature of cellulose digestion by the predominant cellulolytic bacteria in the rumen, trans- Why Don’t Ruminal Bacteria Digest Cellulose Faster? 1% J Dairy Sci 79:149€-1502 1496 Received June 27, 1995. Accepted March 26, 1996. ’Address all correspondence to Paul J. Weimer, US Dairy Forage Research Center, USDA-AM, 1925 Linden Drive West, Madison. WI 53706. PAUL J. WEIMER’ US Dairy Forage Research Center, USDA-ARS, and Department of Bacteriology, University of Wisconsin-Madison, Madison 53706 fer of cellulolytic capabilities to noncellulolytic rumi- nal bacteria (e.g., by genetic engineering) that dis- play other desirable properties offers limited opportu- nities to improve ruminal digestion of cellulose. ( Key words: rumen, digestion, cellulolytic microbes, fiber) Abbreviation key: RCB = ruminal cellulolytic bac- teria. INTRODUCTION Because cellulose is the most abundant component of plant cell walls, ruminal cellulolytic microorgan- isms play a central role in the nutrition of ruminant animals fed diets based on forage. Recognition of this fact has stimulated decades of investigation into the physiology and biochemistry of these microbes. However, only within the last few years has a suffi-
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Paper-Lecture32 - Why Don't Ruminal Bacteria Digest...

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