Synchronized populations of Escherichia coli using simplified self-cycling fermentation

Self-cycling fermentation (SCF) was developed as a method to continuously produce synchronized microbial populations at high cell densities. The present study demonstrates the application of this process to populations of Escherichia coli, making use of a simpler and non-intrusive approach to the control strategy. The carbon dioxide evolution rate (CER) could be easily monitored during growth in batch experiments and its first derivative was found to be a good indicator of the transition from exponential growth to stationary phase. This, in turn, was shown to be a suitable control parameter for stable SCF operation. Earlier approaches to the volume changes during the harvest and refilling steps of SCF using load cells and intermediate vessels have been cumbersome and subject to errors in these crucial measurements. In the improved set up, these were replaced with electro-optic level sensors, which resulted in <2% error in volume. This helped to generate stable synchrony with a high degree of inter-cycle reproducibility. The new system was more robust, reliable, cheaper, not subject to probe fouling and easier to scale up. The cultures of E. coli demonstrated significant synchrony as early as the third cycle and, in every experiment, stable synchrony was observed by the sixth cycle. The stable pattern had a synchrony index between 0.68 and 0.74 and the cycle time was 2.47h+/-0.08. Unexpectedly, the cells doubled in the middle of each synchronized cycle and the length of these cycles was noticeably longer than the doubling time calculated from a batch culture. Neither of these led to a significant loss of cell productivity.