Weimer P J
US Dairy Forage Research Center, USDA-ARS, Madison, WI 53706, USA.
J Dairy Sci. 1996 Aug;79(8):1496-502. doi: 10.3168/jds.S0022-0302(96)76509-8.
The bacteria Fibrobacter succinogenes, Ruminococcus flavefaciens, 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 numerous proposals to improve microbial cellulose digestion in ruminants, it is instructive to examine the characteristics of these species that contribute to their superior cellulolytic capabilities and to identify the factors 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 digestion (first-order rate constants of 0.05 to 0.08 h-1). Active cellulose digestion involves adherence of cells to the fibers via a glycoprotein glycocalyx, which protects cells from protozoal grazing and cellulolytic enzymes 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 species, but overall provide an enormous selective advantage to these cellulolytic bacteria in the ruminal environment. 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, transfer of cellulolytic capabilities to noncellulolytic ruminal bacteria (e.g., by genetic engineering) that display other desirable properties offers limited opportunities to improve ruminal digestion of cellulose.
琥珀酸纤维杆菌、黄化瘤胃球菌和白色瘤胃球菌通常被认为是瘤胃中主要的纤维素分解微生物。对文献中现有数据的比较表明，这些细菌是迄今为止从任何生境中描述的所有嗜温生物中纤维素分解活性最高的。鉴于有许多关于改善反刍动物微生物纤维素消化的提议，研究这些物种有助于其卓越纤维素分解能力的特性，并确定阻碍它们更快消化纤维素的因素是很有意义的。作为一个群体，这些物种具有极端的营养专一性。它们能够利用纤维素（或在某些情况下利用木聚糖）及其水解产物作为其几乎唯一的生长能量来源。此外，每个物种显然已经进化到相似的最大纤维素消化速率（一级速率常数为0.05至0.08 h-1）。活跃的纤维素消化涉及细胞通过糖蛋白糖萼附着在纤维上，这保护细胞免受原生动物的吞噬，并保护纤维素分解酶不被瘤胃蛋白酶降解，同时它至少暂时保留纤维二糖产物供纤维素分解细菌使用。这些特性导致每个物种的附着和未附着群体具有不同的生态作用，但总体上为这些纤维素分解细菌在瘤胃环境中提供了巨大的选择优势。然而，纤维素消化的主要限制因素是植物的细胞壁结构（细胞壁生物聚合物之间的基质相互作用和低底物表面积）以及非运动性纤维素分解微生物对细胞腔的有限渗透。由于这些限制以及瘤胃中主要纤维素分解细菌对纤维素消化的高度适应性，将纤维素分解能力转移到具有其他理想特性的非纤维素分解瘤胃细菌（例如通过基因工程）上，改善瘤胃纤维素消化的机会有限。
