E.Coli

Enzyme and Microbial Technology 29 (2001) 449 – 455

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Modeling of Escherichia coli growth and acetate formation under different operational conditions
Marıa Jesus Guardia*, Eloy Garcıa Calvo ´ ´
Department of Chemical Engineering. Universidad of Alcala, 28871 Alcala de Henares. Spain Received 25 January 2001; revised 20 June 2001; accepted 10 July 2001Abstract A kinetic model for Escherichia coli growth and acetate formation is presented. Stoichiometric and energy balances reduce the number of kinetic equations to estimate. The formulation results in linear relations which establish the dependence of glucose consumption rate and acetate consumption rate with the independent rates, biomass and acetate formation rate. The proposed model has beenused to simulate batch cultures with initial glucose concentrations between 1 and 19 g/liter, continuous cultures with two different feed concentrations (2 and 5 g/liter) and dilution rates between 0.1 and 1 h 1, and experimental results from other authors. Only one set of parameters obtained in an individual and independent way has been used. The accuracy of the simulations allows a description ofthe complex interaction between substrate, microorganism and acetate which is a product and also a carbon source. The general criterions followed in this work could be a reference in kinetic studies where design and scale up are the main aim. © 2001 Elsevier Science Inc. All rights reserved.
Keywords: Kinetic model; Stoichiometry; Energy balance; Escherichia coli; Acetate formation

1.Introduction Escherichia coli is one of the most frequently used host organisms for recombinant protein production. Under typical aerobic cultivation conditions, large amounts of byproducts such as acetate are produced. The accumulation of acetate results in growth inhibition and reduced productivity of the process. However, if the acetate concentration is low, it is used as a secondary carbon sourceprolonging the growth of the cell [1]. Many studies have been done to understand the acetate formation and now it is known that the specific growth rate [2] and specific glucose uptake rate [3] greatly affect its production. The development of a quantitative model to predict such acetate formation may lead to an increase in productivity but also is of technological importance for the design ofindustrial bioprocesses and to monitor and control the environmental conditions of growth. As in most chemical systems, a simple model that allows the description of fundamental phenomena involved in the process would be most useful. Thus, the industrial use of E. coli requires a
* Corresponding author. Tel.: 34-91-885 5088; fax: 5088. Email address: mariaj.guardia@uah.es (M.J. Guardia). 34-91-885model that takes into account the acetate consumption or production depending on the conditions, rather than define all the internal cell reactions. The latter would imply an increase in the number of parameters and consequent difficulty in obtaining them experimentally. A useful and general description of the Escherichia coli growth should take into account behavior under different carbon sources andthe effect of metabolites on the yield and growth rate, as well as a wide range of experimental conditions. In this work we propose a kinetic model for Escherichia coli growth which is useful in predicting our experimental results with different operating conditions and a wide range of initial substrate concentrations, as well as experimental results of other authors found in the literature.

2.Model definition The kinetic model of Escherichia coli growth is appreciably simplified with the dependence among conversion rates using mass and energy conservation principles. The stoichiometric and energy balances allow one to calculate linear relationships between independent conversion rates and to estimate the dependent rates [4,5]. The resulting set of equations can be expressed in a...