Glucose-limited continuous culture

Figure 7 Evolution of the nitrogen ( ) and ash ( ) content of K. marxianus grown in aerobic glucose-limited continuous culture. (Redrawn from Reference [39] with permission of the author and publisher).

Table 15 Macromolecule content (g per 100 g biomass) during anaerobic glucose-limited continuous cultures of S. cerevisiae CBS 8066 [41],...

A glucose-limited continuous culture was grown aerobically at an initial dilution rate of 0.1 h" and the dilution rate was then at time = 1.5 h shifted to 0.40 h , close to the maximum specific growth rate and far above the critical dilution rate [24]. Ethanol and acetic acid production immediately set in. The biomass concentration decreased due to products formation and to a transient imbalance between the growth rate and the dilution rate. 5.5 hours after the shift-up, and before the new steady state was reached, the dilution rate was decreased to 0.05 h . Cells started to grow on glucose and ethanol. 

Table 20 indicates the cost expressed in mmol ATP for anaerobic glucose-limited continuous culture at D = 0.20 h" on mineral media [41] protein polymerization and solute transport account for 70% of the total cost. The predicted ATP requirement is 38.5 mmol per gram biomass. This value represents the minimal cost predicted from biochemistry and does not take into account sub-optimal energy coupling at the cell level. 

Table 20 ATP requirement for macromolecule synthesis and ion transport for an anaerobic glucose-limited continuous culture at D = 0.20 h" on mineral media ([41], modified for lipid content).

Teich, A., Meyer, S., Lin, H.Y., Andersson, L., Enfors, S.-O., and Neubauer, P. (1999) Growth rate related concentration changes of the starvation response ctS and ppGpp in glucose-limited fed-batch and continuous cultures of Escherichia coli. BiotechnoL Progr., 15, 123-129. 

Hewitt C. J., G. Nebe-von-Caron, A. W. Nienow, and C. M. McFarlane (1999). The use of multi-parameter flow cytometry to compare the physiological response of Escherichia coli W3110 to glucose limitation during batch, fed-batch and continuous culture cultivation, J. Biotechnol, 75, 251-254. 

In batch or continuous cultures of C. acetohutylicum DSM 1731, phosphate limitation (initial phosphate concentrations between 0.62 and 0.74 mM) has a profound effect on enhancing the consumption of glucose and production of solvents (Bahl et al. 1982b). The onset of solvent production coincided with the exhaustion of phosphate in the medium and with the culture pH falling below 5. 

Ramsay et al. [91] were the first to investigate PHB and PHBV production in one-and two-stage continuous cultures. In a one-stage chemostat, R. eutropha DSM 545 accumulated 33% of its dry mass as PHB when fed with a nitrogen-limited medium of glucose and mineral salts. PHBV was produced in similar experiments with A. latus... 

This simple approach is illustrated in Figure 19 for a continuous culture of K. marxianus. The evolution of the biomass yield on glucose is linear for the whole range of oxygen limitation. 

As one of its major interests, a model represents an efficient tool for the kinetic analysis of cellular processes. It is able to account for the main phenomena that may simultaneously control the activities of cells. As such, depending on the culture conditions, composition of the medium and whether there is batch or continuous mode of operation, it can be used first to identify the rate-limiting factors and then to characterize quantitatively their relative importance. For instance, with a model it is possible to evaluate the kinetic effect of a depletion of glucose, glutamine and other amino acids or of an accumulation of ammonia and lactate on the rates of cell growth and death. 

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