METABOLIC SHIFT ANALYSIS AT HIGH CELL DENSITIES

In high cell density cultures it is virtually inevitable that the environment to which the cells are exposed is heterogeneous. Thus, with suspended cultures, individual cells are subject to temporal changes in their environment whereas with aggregated or immobilized cells, the culture can be considered as being farmed by a number of subpopulations, each with its own environmental characteristics. In addition, in a high cell density environment, high concentrations of end products may negatively influence the growth rate. This may result in the selection of organisms with an altered metabolic behaviour or with a decreased sensitivity to the adverse effects of the product. We discuss the consequences of this heterogeneity with regard to carbon source metabolism in view of the ability of many bacterial species to adapt to environmental conditions. Selection of variant organisms was found to occur with Clostridium butyricum when grown for a prolonged time in a medium containing approx. 150 mM glucose. In contrast to the original strain, these variants could sustain a high maximal growth rate in the presence of butyric acid. In addition, they had acquired the capacity to spontaneously form aggregates and were able to carry out a completely solventogenic fermentation. Heterogeneous metabolic activity in aggregated cells is demonstrated with cultures of Lactobacillus laevolacticus, an aggregate-forming lactic acid bacterium that converts glucose completely to D-lactate. By using microelectrodes, we show that the fraction of metabolically active cells decreases with increasing aggregate size: in larger aggregates steep pH gradients occur with the effect that only the outer layer of the aggregate is metabolically active, i.e. contributes to lactic acid formation, whereas with smaller aggregates ah cells remain active. As a result, the net specific lactic acid production rate of the population as a whole is not invariably increased with increased aggregate size.