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Efficiency of Rumen Microbial Growth: Influence of some Theoretical and Experimental Factors on ^YATP¹

University of Illinois², Urbana-Champaign 61801

Abstract

Based on known metabolic pathways involved in the biosynthesis of cell macromolecules, the amount of energy (ATP) required for the formation of microbial cells can be estimated. For a microbial rumen population of mixed species, the estimated Y_ATP (grams dry weight cells formed per mole ATP expended) is 29 to 30 for growth on rich media containing preformed monomers and is 20 to 29 for growth on simple media containing carbohydrate and inorganic salts. The ranges in these values result from changes in the composition of the cells formed and not from changes in efficiencies of growth per se. Assuming equal amounts of growth in the rumen occur on rich and simple media conditions, the maximal theoretical Y_ATP would be 26 and this might be lowered to 21, if large allowances are made for ATP expenditures for transport processes. Experimentally determined Y_ATP values often are considerably lower than these theoretical values due to the influence of ATP expenditures for maintenance during growth and variations of Y_ATP with growth rate. The data from nutrient-limited continuous culture (chemostat) studies of single or mixed species indicate rumen bacteria have the potential for a low growth maintenance requirement (M_e) and high cell yields (
of 19 to 20). However, cell yields become progressively lower with decreasing growth rate, increasing M_e, or both as a greater proportion of the ATP produced is diverted to maintenance functions. M_e varies with both bacterial species and growth conditions and is composed of numerous components such as ATP expenditures or losses for motility, turnover, extracellular polymer formations, transport, or energetic uncoupling. A limitation of biosynthetic functions relative to ATP production by catabolism results in both increased energetic uncoupling and M_e, but lower cell yields. This situation can occur readily with bacteria, particularly when biosynthesis is restricted by availability of N sources. Data on rumen NH₃ levels and on NH₃-affinity constants of rumen bacteria indicate that with many diets the prevailing rumen NH₃ levels would seldom limit growth per se via energetic uncoupling. Concentrations of amino acids, oligopeptides, and (or) branched chain volatile fatty acids in the rumen 4 hr or more postfeeding often may be too low to permit good growth, resulting in higher Me via energetic uncoupling and in reduced cell yields. Overall, in vivo factors such as availability of the above nutrients are the major influences on bacterial cell yields as in vitro data indicate the rumen bacterial population as a whole has metabolic potential for efficient growth.

¹ Presented during the 1978 ASAS-ADSA meetings held at Michigan State University, East Lansing, as part of the Ruminant Nutrition Symposium on "Quantitative Aspects of Nitrogen Metabolism in the Rumen."

² Depts. of Dairy Science and Microbiology.

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