Physiological responses to mixing in large scale bioreactors

Escherichia coli fed-batch cultivations at 22 m(3) scale were compared to c orresponding laboratory scale processes and cultivations using a scale-down reactor furnished with a high-glucose concentration zone to mimic the cond itions in a feed zone of the large bioreactor. Formate accumulated in the l arge reactor, indicating the existence of oxygen limitation zones. It is su ggested that the reduced biomass yield at large scale partly is due to repe ated production/reassimilation of acetate from overflow metabolism and mixe d acid fermentation products due to local moving zones with oxygen limitati on. The conditions that generated mixed-acid fermentation in the scale-down reactor also induced a number of stress responses, monitored by analysis o f mRNA of selected stress induced genes. The stress responses were relaxed when the cells returned to the substrate limited and oxygen sufficient comp artment of the reactor. Corresponding analysis in the large reactor showed that the concentration of mRNA of four stress induced genes was lowest at t he sampling port most distant from the feed zone. It is assumed that repeat ed induction/relaxation of stress responses in a large bioreactor may contr ibute to altered physiological properties of the cells grown in large-scale bioreactor. Flow cytometric analysis revealed reduced damage with respect to cytoplasmic membrane potential and integrity in cells grown in the dynam ic environments of the large scale reactor and the scale-down reactor. (C) 2001 Elsevier Science B.V. All rights reserved.