Living With Heterogeneities in Bioreactors: Understanding the Effects of Environmental Gradients on Cells

Abstract

The presence of spatial gradients in fundamental culture parameters, such as dissolved gases, pH, concentration of substrates, and shear rate, among others, is an important problem that frequently occurs in large-scale bioreactors. This problem is caused by a deficient mixing that results from limitations inherent to traditional scale-up methods and practical constraints during large-scale bioreactor design and operation. When cultured in a heterogeneous environment, cells are continuously exposed to fluctuating conditions as they travel through the various zones of a bioreactor. Such fluctuations can affect cell metabolism, yields, and quality of the products of interest. In this review, the theoretical analyses that predict the existence of environmental gradients in bioreactors and their experimental confirmation are reviewed. The origins of gradients in common culture parameters and their effects on various organisms of biotechnological importance are discussed. In particular, studies based on the scale-down methodology, a convenient tool for assessing the effect of environmental heterogeneities, are surveyed.

Metabolic Flux Analysis of Escherichia coli creB and arcA Mutants Reveals Shared Control of Carbon Catabolism under Microaerobic Growth Conditions

“…Such restricted conditions of carbon and oxygen supply exist in nature in certain environmental niches and occur widely as transient fluctuations that can elicit a selective response from bacterial cells (26). Similar conditions take place in industrial cultures as a consequence of inadequate mixing within the bioreactors, e.g., in operation at high cell density in the fed-batch mode (36).…”

Adaptation and Heterogeneity of Escherichia coli MC1000 Growing in Complex Environments

“…In previous work, locally divergent conditions were suggested to constitute a negative factor for process yields, as bacterial growth and effectiveness were found to be lower in heterogeneous than in homogeneous systems (7). However, in such studies the ability of the bacteria to adapt to environmental conditions (even when they are complex) has been disregarded.…”

Amino acids production focusing on fermentation technologies – A review

“…In a larger scale reactor, the different geometry and physical conditions may affect important parameters leading to a lower process stability, reproducibility and yields and to the formation of unwanted by-products that may affect the final product quality (Takors, 2012). In particular, industrial bioprocesses are often affected by lower mixing efficiency with consequent long mixing times that, combined with the high metabolic activity of microbial cells, results in the formation of local gradients into the bioreactors (Lara et al, 2006). Moreover, this problem is compounded by the increased reactor size.…”

“…The concentration gradients of substrate and oxygen follow an opposite trend along the reactor height. In an industrial fermentor these gradients are more pronounced due to a longer distance to be covered, leading to larger substrate and oxygen depletion zones, larger volumes of culture broth to be stirred thereby longer mixing time as well as stronger hydraulic pressure gradient influencing the oxygen transfer rate (Lara et al, 2006). Therefore, the microorganisms at the top of the fermentor are simultaneously exposed to a high sugar concentration together with oxygen limitations, while the one at the bottom are exposed to glucose restrictions (Schmidt, 2005).Therefore, as a consequence of the combined high glucose concentration and oxygen limitation in the reactor, acetate, ethanol, lactate, hydrogen, succinate and formate are produced in high amounts (Castan and Enfors, 2001).…”