Cybernetic Modeling of Growth in Mixed, Substitutable Substrate E n vir o n m en t s zyxwv : Preferen t i a I a n d Simultaneous Utilization Ramprasad Ramakrishna and Doraiswami Ramkrishna* zyxwv School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907 Allan E. Konopka Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907 Received June zyxwvutsrq 1, 1995/Accepted March 2 zyxwvut I, 1996 Growth of microorganisms on substitutable substrate mixtures display diverse growth dynamics characterized by simultaneous or preferential uptake of carbon sources. This article shows that cybernetic modeling concepts which were successful in predicting diauxic growth pat- terns can be extended to describe simultaneous con- sumption of substrates. Thus the growth of Escherichia coli on mixtures of glucose and organic acids such as pyruvate, fumarate, and succinate has been described successfully by the cybernetic model presented here showing both diauxic and simultaneous uptake when ob- served. The model also describes the changes in utiliza- tion patterns that occur under changing dilution rates, substrate concentrations, and models of preculturing. The model recognizes the importance of the synthesis of biosynthetic precursors in cell growth through a kinetic structure that is quite general for any mixture of carbon- energy sources. @ 1996 John Wiley & Sons, Inc. Key words: cybernetic modeling biosynthetic precursors metabolic regulation enzyme synthesis INTRODUCTION zyxwvutsrq The performance of most biochemical processes medi- ated by microorganisms, such as fermentations and bio- degradation, is influenced by the dynamics of microbial growth in multiple nutrient-limited environments. The nutrient limitation may include sets of substitutable sub- strates that is, those fulfilling identical growth require- ments, or complementary ones where each substrate fulfills an essential growth requirement. More generally, both types of substrate combinations may be present. The vast majority of studies of growth in mixed substi- tutable substrate cultures have been with carbon-energy sources and reveal a spectrum of growth dynamics and substrate utilization patterns. These situations are fre- quently encountered in fermentations that use complex media and in the biodegradation of pollutants. * zyxwvutsrqpon To whom all correspondence should be addressed. Telephone: (317)494-4066, fax: (317)494-0805; e-mail ramkrish@ecn.purdue.edu Mathematical models which possess predictive capa- bilities to describe these dynamics are invaluable in the rational design of bioprocesses. In the literature, there are several modeling efforts that attempt to describe mixed-substrate growth dynamics. These models are only able to predict diauxic growth and cannot describe simultaneous utilization of the substrates. Unstructured models based on modified forms of the Monod equation, such as the model of Yoon et al. (1977), are inadequate for describing varied growth patterns that arise from complex regulatory processes. The models of van Dedem and Moo-Young (1975) and Nikolajsen et al. (1991), which are based on structured representations of a cell and do include some aspects of metabolic regulation, however incorporate specific biochemical features that constrain their application to the system under consideration. However, the cyber- netic model of Kompala et al. (1986), also based on a structured representation of the cell, represents the control of the important biochemical processes incorpo- rating no new parameters through the recognition of the optimal nature of microbial growth on mixed substrates. This kinetic structure and description of regulation is general to any combination of substitutable substrates and describes mixed-substrate growth with parameters determined from single-substrate experiments. The model, discussed in more detail under Cybernetic Mod- eling, is however unable to describe simultaneous utili- zation of substrates during mixed-substrate growth, due to the rigidity of the kinetic structure. We have developed a model with an expanded kinetic structure that uses the same cybernetic principles as the model of Kompala and considers the formation of key biosynthetic precursors and enzymes to describe a more complete spectrum of mixed-substrate growth dynamics observed in the literature. The model describes diauxic growth as well as simultaneous substrate uptake and Biotechnology and Bioengineering, Vol. 52, Pp. 141-151 (1996) 0 1996 John Wiley & Sons, Inc. CCC 0006-3592/96/010141-11