Parallel substrate supply and pH stabilization for optimal screening of E. coli with the membrane-based fed-batch shake flask.

BACKGROUND

Screening in the fed-batch operation mode is essential for biological cultivations facing challenges as oxygen limitation, osmotic inhibition, catabolite repression, substrate inhibition or overflow metabolism. As a screening tool on shake flask level, the membrane-based fed-batch shake flask was developed. While a controlled supply of a substrate was realized with the in-built membrane tip, the possibilities for replenishing nutrients and stabilizing pH values was not yet exploited. High buffer concentrations were initially used, shifting the medium osmolality out of the biological optimum. As the growth rate is predefined by the glucose release kinetics from the reservoir, the resulting medium acidification can be compensated with a controlled continuous supply of an alkaline compound. The focus of this research is to establish a simultaneous multi-component release of glucose and an alkaline compound from the reservoir to enable cultivations within the optimal physiological range of Escherichia coli.

RESULTS

In combination with the Respiratory Activity MOnitoring System, the membrane-based fed-batch shake flask enabled the detection of an ammonium limitation. The multi-component release of ammonium carbonate along with glucose from the reservoir resulted not only in the replenishment of the nitrogen source but also in the stabilization of the pH value in the culture medium. A biomass concentration up to 25 g/L was achieved, which is one of the highest values obtained so far to the best of the author's knowledge with the utilization of a shake flask and a defined synthetic medium. Going a step further, the pH stabilization allowed the decrease of the required buffer amount to one-fourth establishing an optimal osmolality range for cultivation. As optimal physiological conditions were implemented with the multi-component release fed-batch cultivation, the supply of 0.2 g glucose in a 10 mL initial culture medium volume with 50 mM MOPS buffer resulted in a twofold higher biomass concentration than in a comparable batch cultivation.

CONCLUSIONS

The newly introduced multi-component release with the membrane-based fed-batch shake flask serves a threefold purpose of replenishing depleted substrates in the culture medium, stabilizing the pH throughout the entire cultivation time and minimizing the necessary amount of buffer to maintain an optimal osmolality range. In comparison to a batch cultivation, these settings enable to achieve higher biomass and product concentrations.