Chapter 18
Cellulose Degradation by Ruminai Microbes: Physiological and Hydrolytic Diversity Among Ruminai Cellulolytic Bacteria Paul J. Weimer
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1
1,2
and Christine L. Odt
1
U.S. Dairy Forage Research Center, Agricultural Research Service, U.S. Department of Agriculture, Madison, WI 53706 Department of Bacteriology, University of Wisconsin, Madison, WI 53706 2
Cellulose fermentation by ruminai microorganisms provides a major contribution to the nutrition of forage-fed ruminant animals. Ruminai cellulose degradation is mediated primarily by a few anaerobic bacterial species that avidly attach to forage particles in the rumen and degrade the cellulose components by cell-bound complexes of cellulolytic enzymes. The rate of cellulose degradation is largely a function of the available surface area of the cellulose. Crystallinity of the cellulose appears to be relatively unimportant in ruminal cellulose digestion, but evaluation of crystallinity effects is complicated by artifacts associated crystallinity measurements. Individual species vary considerably with respect to the attachment process and to utilization of different cellulose allomorphs, suggesting important differences in the mechanisms of cellulose digestion at the enzymatic level. These differences, combined with differences in microbial growth efficiency and product formation, provide both stability and flexibility to fiber digestion in the rumen. The rumen is the definitive digestive organ in the ruminants, a specialized group of mammals (Order Artiodactales) that includes many familiar wild animals (e.g., deer, antelope and bison) as well as some of the more important farm animals (cattle, sheep, and goats). In essence, the rumen is a large fermentation vat in which feeds are fermented by anaerobic microorganisms to produce volatile fatty acids (used by the ruminant as oxidizable energy sources and anabolic precursors) and microbial cell mass (used by the ruminant as a protein source). The combined ruminai volume of the world's 1.1 χ 10 domesticated cattle, 1.0 χ 10 sheep , and 0.4 χ 10 goats (7) is on the order of 10H liters. Thus, the ruminai fermentation may be regarded as the world's largest and most important commercial fermentation process, and attempts to improve its efficiency and productivity can be viewed as a very worthwhile endeavor. 9
9
9
This chapter not subject to U.S. copyright Published 1995 American Chemical Society
Saddler and Penner; Enzymatic Degradation of Insoluble Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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ENZYMATIC DEGRADATION OF INSOLUBLE CARBOHYDRATES
The primary role of ruminants in agriculture is to convert relatively recalcitrant cellulosic materials to useful endproducts (e.g., milk or meat). The role of the rumen microflora in accomplishing this process is facilitated by the relatively long (15-50 h) retention time for fibrous solids and by the many complex interactions among different microbial trophic groups (2). Ruminai digestion of plant fiber is currently thought to be limited primarily by the complex structure of the plant cell wall, particularly the physical protection provided by lignin, and by covalent linkages between lignin and/or phenolic acids and certain cell wall polysaccharides (e.g., arabinoxylans) (3). Cellulose itself appears to be rather easily digestible i f the extensive matrix interactions among plant cell wall biopolymers can be disrupted (3,4). However, ruminai cellulose digestion can hardly be considered as a monolithic process, because of the great variety of ruminai microbial species known to be capable of cellulose hydrolysis. Although the taxonomic and physiological diversity among different cellulolytic groups (protozoa, fungi, and bacteria) has long been recognized, the diversity of the cellulolytic process within individual groups has received relatively little attention. Among the bacteria, generally considered to be the major cellulolytic group within the rumen (2), this diversity presents the potential for alternative mechanisms of cellulose hydrolysis and for complex ecological interactions among the different cellulolytic species. Moreover, the diversity among cellulolytic bacteria and its ecological expression have profound effects on animal nutrition. Physiological Diversity Among the Ruminai Cellulolytic Bacteria The ruminai cellulolytic bacteria may be functionally divided into two groups: those that adhere to plant fiber and those that do not. The nonadherent species are generally considered to be of little importance in ruminai cellulolysis, based on their relatively low numbers in vivo and their poor cellulolytic activity in vitro. Within the other group, comprising the adherent forms, three major species have been identified: Fibrobacter succinogenes, Ruminococcusflavefaciens,and R. albus. These three species share several common properties: all are strictly anaerobic (i.e., will not grow in media containing O 2 or having an elevated redox potential), do not form a protective resting stage, and are nonmotile. In addition, all three are highly specialized catabolically in that they can use only cellulose (or in some cases xylan) and its hydrolytic products as growth substrates. Our interest in these species arose from the fact that they produce different ratios of fermentation endproducts (Table I), and these differences could, in principle, have major effects on animal response. Moreover, differences in certain intrinsic properties of these species (e.g., Gram reaction) suggest structural and biochemical differences whose manipulation could provide a possible means of regulating their populations in rumino. For example, selective in rumino enhancement of F. succinogenes over the ruminococci might be desirable for beef cattle or for heifers (young cows prior to their first breeding and lactation). This enhancement would favor production of succinate, which in rumino is converted by other bacteria to propionate, which in turn is absorbed by the animal and used for gluconeogenesis that is essential for animal live weight gain. On the other hand,
Saddler and Penner; Enzymatic Degradation of Insoluble Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
18. WEIMER AND ODT
Cellulose Degradation by Ruminai Microbes
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selective enhancement of the ruminococci might be desirable in lactating cows to provide the acetate needed for lipogenesis in the mammary gland, in order to produce milk with a sufficient content of fat for end uses such as cheese production.
Downloaded by FUDAN UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0618.ch018
Table I. Characteristics of adherent species of ruminai cellulolytic bacteria Fibrobacter RuminococcusRuminococcus albus Parameter succinogenes flavefaciens + Gram reaction + Growth substrates Cellulose Cellodextrins Glucose Xylan Fermentation products Succinate Acetate Formate Lactate Ethanol Hydrogen a
b
c
d
+ + +
+ +
+ + -/+ a
+
+
-
-
m M yd m/-
-
yd
-b
c
M m m
a
M m m M M
varies with strain xylan hydrolyzed, but hydrolytic products not utilized M= major product (>1 mol/mol anhydroglucose consumed); m=minor product (
