(623f) Biorefining Mixed Sugars Using High Densities of Growth-Arrested Corynebacteria

Research and development of renewable energies has recently regained prominence given anticipated shortages of fossil-fuel-based energies and parallel rising prices of fossil-derived fuels and chemicals.

Particularly, the quantities of ethanol produced via biotechnological processes to displace petroleum as a transportation fuel are expected to dramatically increase in the coming years. At present, the bulk of the industrial ethanol production is ensured via the fermentation of easily accessible sugars, for example derived from corn starch, sugar cane or sugar beet. The yeast Saccharomyces cerevisiae is the primary organism to ensure these fermentations. However, the cost analysis of the existing production processes clearly demonstrates that their economic performances are tightly linked to the costs of fermentation raw materials. Furthermore, the flattening experience curve observed by industrial manufacturers suggests that radical innovation is needed to make bio-ethanol a competitive commodity compared to fossil fuels. As a result, it becomes increasingly clear that the global scale implementation of ethanol as a primary transportation fuel will only become a reality once sugars can be extracted from abundant and easily accessible lignocellulosic biomass in a cost-effective manner. However, one of the current technological limitations of yeast-based processes is that they are significantly negatively impacted by the presence of various growth inhibitors that are typically generated at the pre-treatment step of the saccharification process of lignocellulosic materials.

We have previously validated the concept of decoupling the cell generation phase and the product production phase of fermentation processes, as a means to enable the use of conditions that are sub-optimal for cellular growth but optimal for product production. Moreover, we proposed a process that is characterized by a minimal number of manufacturing steps and that can be applied to anaerotolerant organisms, such as Corynebacterium glutamicum, or quorum sensing sensitive organisms such as Escherichia coli. This flexibility and simplicity in design represent important manufacturing advantages that combine with the proven robustness of selected industrial microorganisms to enable bioconversions characterized by favourable economics. The intrinsic robustness of the design we propose is a key feature for capturing synergies between various production processes, and thus making possible the optimum manufacturing integration within a biorefinery framework.

Furthermore, we have engineered the amino acid production workhorse C. glutamicum to produce ethanol via continuous or semi-continuous processes that operate at very high cellular densities and that do not require concomitant cellular growth¹⁾.

Notably, the strain/bioprocess couples we developed using these technologies exhibit two critical attributes: on the one hand a greatly reduced negative impact on yields of various growth-inhibitors that are typically present in saccharified mixtures of lignocellulosic biomass materials, and on the other hand the parallel utilisation of glucose and xylose. Moreover, essentially the same process can be applied to produce the organic acids lactate and succinate^2,3) at very high efficiencies.

This study was partially supported by a grant from New Energy and Industrial Technology Development Organization (NEDO).

1)M. Inui et al., J. Mol. Microbiol. Biotechnol. 8:243-254. 2004. 2)M. Inui et al., J. Mol. Microbiol. Biotechnol. 7:182-196. 2004. 3)S. Okino et al., Appl. Microbiol. Biotechnol. 68:475-480 2005.