WHY DONT RUMINAL BACTERIA DIGEST CELLULOSE FASTER

The bacteria Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus generally are regarded as the predominant celluloly tic microbes in the rumen. Comparison of available data from the liter ature reveals that these bacteria are the most actively cellulolytic o f all mesophilic organisms described to date from any habitat. In ligh t of numerous proposals to improve microbial cellulose digestion in ru minants, it is instructive to examine the characteristics of these spe cies that contribute to their superior cellulolytic capabilities and t o identify the factors that prevent them from digesting cellulose even more rapidly. As a group, these species have extreme nutritional spec ialization. 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 maxim um rates of cellulose digestion (first-order rate constants of 0.05 to 0.08 h(-1)). Active cellulose digestion involves adherence of cells t o 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 prod ucts for use by the cellulolytic bacteria. These properties result in different ecological roles for the adherent and nonadherent population s of each species, but overall provide an enormous selective advantage to these cellulolytic bacteria in the ruminal environment. However, m ajor constraints to cellulose digestion are caused by cell-wall struct ure of the plant (matrix interactions among wall biopolymers and low s ubstrate surface area) and by limited penetration of the nonmotile cel lulolytic microbes into the cell lumen. Because of these constraints a nd the highly adapted nature of cellulose digestion by the predominant cellulolytic bacteria in the rumen, transfer of cellulolytic capabili ties to noncellulolytic ruminal bacteria (e.g., by genetic engineering ) that display other desirable properties offers limited opportunities to improve ruminal digestion of cellulose.