The role of ruminal carboxymethylcellulases in the degradation of β-glucans from cereal grain

Abstract

The non-cellulolytic ruminal bacterium, Prevotella bryantii, grew rapidly on water soluble β-glucans, but a mutant deficient in carboxymethylcellulase (CMCase) activity could not. Native activity gels prepared with CMC and β-glucan indicated that the CMCase and β-glucanase activities migrated at similar rates. When a plasmid carrying the P. bryantii CMCase was transferred to Escherichia coli, the clone had CMCase and β-glucanase activities. P. bryantii grew on mixed β-1,3-1,4 glucans, but not on β-1,3 glucan, and similar results were obtained with the cellulolytic ruminal bacteria ( Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD1 and Ruminococcus albus B199). Mixed ruminal bacteria from cattle fed hay had twice as much CMCase activity as bacteria from cattle fed 90% cereal grain ( P<0.05), and the CMCase and β-glucanase activities were highly correlated ( r 2=0.93) for the 22 samples tested. The CMCase and β-glucanase activities of mixed ruminal bacteria migrated slowly through polyacrylamide gels, but the migration distances were approximately the same. When β-glucan-utilizing ruminal bacteria were isolated from cattle fed hay or 90% cereal grain, 70 and 38% of the strains, respectively, had CMCase activity. A similar trend was observed with cellobiose-utilizing isolates (70 and 35%, respectively, were CMCase positive). All CMCase positive, cellobiose-utilizing ruminal bacteria could grow on β-glucan. CMCase activity was not strongly correlated with cellulose utilization, and less than 15% of the CMCase positive isolates grew on ball-milled cellulose. Based on these results, the cell-associated CMCases of ruminal bacteria provide a mechanism for utilizing water soluble β-glucans from cereal grains.