Simulation of dissolved CO(2) gradients in a scale-down system: a metabolic and transcriptional study of recombinant...

Deficient mixing in industrial-scale bioreactors is an important concern as it results in a heterogeneous environment that may affect microbial cell physiology. Dissolved carbon dioxide (dCO(2) ) fluctuations, which can occur in large-scale bioreactors, were simulated for the first time and their effects were evaluated on Escherichia coli expressing recombinant green fluorescent protein (GFP). The dCO(2) gradients were simulated by continuously circulating the medium between two vessels of a scale-down system to mimic mean circulation times (t(c) ) of 50, 170, and 375 s. Specific growth rate (μ) decreased by 11% and acetate concentration increased by 23% at the highest t(c) compared to reference cultures. An increase in the time needed for attaining maximum GFP concentration was also observed. The effect of dCO(2) fluctuations on the transcriptional levels of genes involved in the glutamate decarboxylase (gadA and gadC) and α-ketoglutarate dehydrogenase (sucA and sucB) were analyzed by quantitative real time PCR. Such genes are known to be highly over- or underexpressed at elevated constant dCO(2) . Expression levels of gadA and gadC increased up to 60% and 72%, and sucA decreased 1.8-fold in the culture performed at the highest t(c) . Only a minor decrease of sucB expression was observed at t(c) of 170 and 375 s. Although exposure to continuous high dCO(2) can affect culture performance, in this work it was shown that E. coli is able to rescue its metabolism in very short times when cells are intermittently returned to low dCO(2) conditions after being exposed to high dCO(2) .