Comparison of Maintenance Energy Expenditures and Growth Yields Among Several Rumen Bacteria Grown on Continuous Culture

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Abstract
Maintenance energy expenditures were measured for 5 rumen bacteria, Selenomonas ruminantium, Butyrivibrio fibrisolvens, Bacteroides ruminicola, Megasphaera elsdenii and Streptococcus bovis, by using a complex medium with glucose as the C source. Large differences (as high as 8.5-fold) in maintenance energy expenditures were seen among these bacteria. The suggestion is made that maintenance requirements could be a significant determinant of bacterial competition in the rumen. Theoretical maximum growth yields, calculated from double reciprocal plots of yield vs. dilution rate, were compared to theoretical YATPmax [maximum molar ATP yield] values to estimate minimum molar ATP yields from glucose for each bacterium. Results showed that relative yield among the bacteria was growth rate dependent. At high dilution rates, both S. ruminantium and S. bovis produced lactate as their principal fermentation product. At lower dilution rates very little lactate was formed and growth yields increased. Acetate and ethanol were the predominant fermentation products of S. bovis at low dilution rates. Other workers showed that S. ruminantium produces acetate and propionate at low growth rates.