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Physiological responses to mixing in large scale bioreactors
S O Enfors1, M Jahic, A Rozkov
1Department of Biotechnology, Royal Institute of Technology, S-10044, Stockholm, Sweden. enfors@biotech.kth.se
Journal of Biotechnology
|February 13, 2001
Summary
Large-scale Escherichia coli cultivations show reduced biomass yield due to localized oxygen limitation zones, causing stress responses and altered cell physiology. Scale-down reactors mimic these conditions, revealing insights into bioreactor performance.
Area of Science:
- Biotechnology
- Microbial Physiology
- Bioprocess Engineering
Background:
- Fed-batch cultivations of Escherichia coli are crucial for industrial biotechnology.
- Scaling up bioreactor processes often leads to reduced biomass yield and altered cell physiology.
- Localized zones of oxygen limitation and nutrient gradients occur in large-scale bioreactors.
Purpose of the Study:
- To compare Escherichia coli fed-batch cultivations at large scale (22 m3) with laboratory scale and scale-down reactor models.
- To investigate the impact of localized oxygen limitation and nutrient gradients on cell physiology and gene expression.
- To understand the causes of reduced biomass yield in large-scale bioreactors.
Main Methods:
- Comparison of 22 m3 fed-batch cultivations with laboratory-scale processes.
- Utilizing a scale-down reactor with a high-glucose concentration zone to mimic large bioreactor feed zones.
- Monitoring gene expression (mRNA) of stress-induced genes and analyzing cell membrane integrity via flow cytometry.
Main Results:
- Formate accumulation in the large reactor indicated oxygen limitation zones.
- Reduced biomass yield at large scale is linked to acetate production/re-assimilation from overflow metabolism and mixed acid fermentation.
- Scale-down reactor conditions induced stress responses in E. coli, which relaxed upon return to optimal conditions.
- mRNA levels of stress-induced genes were lowest furthest from the feed zone in the large reactor.
- Cells in dynamic environments (large and scale-down reactors) showed reduced cytoplasmic membrane damage.
Conclusions:
- Localized oxygen limitation and associated metabolic shifts (overflow metabolism, mixed acid fermentation) contribute to reduced biomass yield in large-scale E. coli cultivations.
- Repeated induction and relaxation of stress responses in large bioreactors can alter cellular physiology.
- Scale-down models effectively mimic critical conditions in large bioreactors, providing valuable insights for process optimization.