Microbial growth in continuous cultures subject to single and multiple limitations involving carbon and/or nitrogen

Jeffrey Vincent Straight, Purdue University

Abstract

Mathematical models which utilize standard reaction kinetics to describe microbial growth are generally successful under carbon-limiting conditions in batch and steady-state continuous cultures. However, when a class of substrate other than the available carbon source is limiting even these models fail in predicting microbial behavior under conditions in which they were previously successful. Regardless of the substrate limitation standard kinetic models also poorly represent microbial dynamics exhibited under transient conditions in continuous cultures since they largely ignore the significance of cellular regulatory processes. Therefore, such models are incapable of describing the transitions between different metabolic pathways when limiting substrates are complementary as well as during transient conditions. The need for a versatile and robust modeling framework to describe microbial behavior when limited by complementary as well as substitutable substrates is clearly present. By combining the ideals of the cybernetic framework with a topological perspective of metabolic pathways this work develops an expanded framework which overcomes the previous difficulties in describing the utilization of complementary substrates while retaining the original results proposed by Ramkrishna and co-workers to describe the utilization of substitutable substrates. The growth characteristics of the bacterium Escherichia coli W are experimentally studied in steady-state and transient continuous cultures. Three limiting conditions are investigated: carbon-limiting, nitrogen-limiting, and dual-limiting. Glucose serves as the required carbon source while NH$\sbsp{4}{+}$ supplies the necessary nitrogen. The proposed model quantitatively describes the results under singly-limiting conditions and accurately identifies the limiting substrate as well as the substrate present in excess. All of the model parameters are estimated from results under singly-limiting conditions or results available in the literature. Without any additional modifications the proposed model is also applied to cultures which are dual-limited by both carbon and nitrogen. The model accurately predicts the occurrence of a transition between the carbon- and nitrogen-limiting conditions. Overall, the agreement between the model simulations and the experimental results under dual-limiting conditions is quantitative as well as qualitative. Operating a fermenter under any one of the three limiting conditions offers distinct advantages which are due primarily to the presence of metabolic regulation. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

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