Initial pH as a Determinant of Cellulose Digestion Rate by Mixed Ruminal Microorganisms In Vitro

• Published in: Journal of dairy science Vol. 84; no. 4; pp. 848 - 859
• Main Authors: Mourino, F, Akkarawongsa, R, Weimer, P. J
• Format: Journal Article
• Language: English
• Published: Savoy, IL Am Dairy Sci Assoc 01.04.2001; American Dairy Science Association
• Subjects: Animal productions; Animal, plant and microbial ecology; Animals; Bacteria - metabolism; Bacterial Adhesion; Biological and medical sciences; Cattle; Cellulose - metabolism; Cellulose - pharmacokinetics; Digestion; Female; Fermentation; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; In Vitro Techniques; Microbial ecology; Normal microflora of man and animals. Rumen; Rumen - metabolism; Rumen - microbiology; Terrestrial animal productions; Vertebrates; Microbial activity; Farming animal; Nutrition; Ruminant animal; Cellulose; Rumen; Digestion; Experimental study; Fermentation; Microorganism culture; pH; Associated microflora; Simulation model; Rate constant
• ISSN: 0022-0302; 1525-3198
• DOI: 10.3168/jds.s0022-0302(01)74543-2

In vitro fermentations of pure cellulose by mixed ruminal microorganisms were conducted under conditions in which pH declined within ranges similar to those observed in the rumen. At low cellulose concentrations (12.5 g/L), the first-order rate constants (k) of cellulose disappearance were successively lower at initial pH values of 6.86, 6.56, and 6.02, but in each case the value of k was maintained over a pH range of 0.3 to 1.2 units, as the fermentation progressed. Plots of k versus initial pH were linear, and k displayed a relative decrease of approximately 7% per 0.1 unit decrease in pH. At high cellulose concentration (50 g/L) and an initial pH of 6.8, cellulose digestion was initially zero order, the absolute rate of digestion declined with pH and digestion essentially ceased at pH 5.3 after only 30% of the added cellulose was digested. Further incubation resulted in a loss of bound N and P, suggesting that at low pH cells lysed or detached from the undigested fibers. Pure cultures of ruminal cellulolytic bacteria also were able to ferment cellulose to a minimum pH of 5.1 to 5.3, but the extent of fermentation was increased by coculture with noncellulolytic bacteria. A model is proposed in which the first-order rate constant of cellulose digestion is determined by the pH at which the fermentation is initiated, and end product ratios reflect greater activity of the noncellulolytic population as pH declines.