Cybernetic modeling of bacterial metabolite production

Matthew Lucian Alexander, Purdue University

Abstract

The class of bacterial metabolites known as siderophores (iron-chelating agents) presents an interesting situation in metabolic regulation. Siderophores are synthesized under iron-limiting conditions, and act to chelate extracellular iron and transport it into the cell. The cybernetic modeling framework, which seeks to describe complex metabolic regulation in a simple manner, was used in this work to investigate its general applicability to such a case of bacterial metabolite production. The siderophore-mediated transport of iron into the cell represents a high affinity uptake process, while a low affinity uptake process also occurs at higher iron levels. Since iron is a required micronutrient for most microorganisms, the effect of various levels of iron on growth characteristics added a further dimension to the experimental work. Fermentations were carried out using Escherichia coli, which produces the siderophore enterochelin, in both batch and continuous (steady state) operating modes, over a 250-fold range of iron levels. Higher siderophore levels in conditions of greater iron limitation were observed, along with a constitutive production of siderphore in iron-sufficient conditions. Also, growth rates and observed yields on carbon decreased with greater iron limitation. A cybernetic model was developed to describe this situation of carbon- and iron-limited growth along with siderophore production. The specific processes included in the model were siderophore synthesis, iron transport by both the low and high affinity processes, production of a cellular energy resource in which iron is catalytically involved, and resource-dependent growth. The metabolic regulation of these processes was implemented using cybernetic (control) variables. Simulation results from this model agreed quantitatively with the experimental data of the four components followed, namely biomass, siderophore, medium and cell iron. A simple representation of the energy resource generation process allowed for a good quantitative description of both the decrease in growth rate and yield on carbon, as represented by biomass levels. Finally, the inclusion of both transport processes yielded an overall uptake of iron by the cell which agreed very well over a very broad range of medium iron levels and growth rates.

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

COinS