Effects of Protein, Carbohydrate, and Fat Sources on Bacterial Colonization and Degradation of Fiber In Vitro

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Abstract
In trial 1, our objectives were to study effects of different substrates (cellulose, red clover, and orchardgrass) on bacterial colonization and degradation of fiber. To quantitate bacterial colonization, we used 15N as a marker. Use of 15N appeared to underestimate bacterial colonization of cellulose, but it was assumed that relative differences among treatments and across times were accurate. The 15N and carboxymethylcellulase activity techniques gave similar patterns for bacterial colonization with time on purified cellulose but not orchardgrass or red clover; this indicated a higher concentration of cellulolytic versus total bacteria colonizing cellulose. Relatively lower detachment from red clover or orchardgrass than cellulose with time may have been due to selection for different types of microbes that were attached more firmly or were less prone to lysis. In trial 2, replacing cellulose with 30% starch or different protein sources (12% CP) decreased NDF digestion of crystalline cellulose but increased adherent bacterial CP concentration (estimated using 15N) and carboxymethylcellulase activity. The addition of starch and preformed protein may have selected for adherent, noncellulolytic microbes and decreased cellulolysis. The addition of 10% unsaturated or saturated fat did not affect colonization or NDF digestion, perhaps because of the larger surface area of the cellulose dispersing fatty acids more than would occur with more typical substrates. The addition of starch probably increased carboxymethylcellulase activity more than when using purines or 15N. Experiments using pure cultures of bacteria or purified substrates are not necessarily related to those using mixed cultures or natural forages.