WHY ARE RUMINAL CELLULOLYTIC BACTERIA UNABLE TO DIGEST CELLULOSE AT LOW PH?

Authors: Russell, James; WILSON, DAVID - CORNELL UNIVERSITY

Submitted to: Journal of Dairy Science
Publication Type: Review Article
Publication Acceptance Date: 3/28/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Not required.

Technical Abstract: Ruminant animals depend on cellulolytic rumen bacteria to digest cellulose, but these bacteria cannot resist the low ruminal pH that modern feeding practices can create. Because the cellulolytics cannot grow on cellobiose at low pH, pH sensitivity is a general aspect of growth and not just a limitation of the cellulases per se. Acid-resistant ruminal bacteria have evolved the capacity to let their intracellular pH decrease, maintain a small pH gradient across the cell membrane (deltapH), and prevent an accumulation of volatile fatty acid anions. Cellulolytic bacteria cannot grow with a low intracellular pH, and an increase in deltapH leads to anion toxicity. Prevotella ruminicola cannot digest native cellulose, but it grows at low pH and degrades the cellulose derivative carboxylmethylcellulose (CMC). The P. ruminicola CMCase cannot bind to cellulose, but a recombinant enzyme having the P. ruminicola catalytic domain and a binding domain from Thermomonspora fusca was able to bind and had at least 10 fold higher cellulase activity. Based on these results, gene reconstruction offers a means of converting P. ruminicola into a ruminal bacterium that will digest cellulose at low pH.