Discussion and Conclusions

As outlined in the earlier reviews and summarized in Table 3, wild-type strains of Z. mobilis (and their mutants) can convert simple sugars to ethanol at faster rates and higher yields compared to yeasts. However, the ethanol industry has traditionally used yeasts, and despite the apparent advantages of Z. mobilis, there appears to be little incentive for change with sugar and

Table 3 Characteristics of Z. mobilis for production of fuel ethanol and higher value products

1. Considerably faster specific rates of sugar uptake and ethanol production (specific rates 2-3 times faster than yeasts).

2. Higher ethanol and lower biomass yields compared to yeasts due to different carbo­hydrate metabolism (Entner-Doudoroff vs. glycolytic pathway).

3. Higher reported productivities (120-200 gL-1 h-1) in continuous processes with cell recycle (maximum reported values for yeasts are 30-40 gL-1 h-1).

4. Simpler growth conditions. Z. mobilis grows anaerobically (not strict anaerobe) and does not require the controlled addition of oxygen to maintain cell viability at high ethanol concentrations.

5. Ethanol tolerance comparable if not better than yeasts. Ethanol concentrations of 85gL-1 (11% v/v) reported for continuous culture and up to 127gL-1 (16% v/v) in batch culture.

6. Laboratory scale studies with strains of Z. mobilis over many years in controlled fer­mentations (pH = 5.0, T = 30 ° C) have not revealed any significant contamination or bacteriophage infection problems.

7. The wide range of techniques developed for the genetic manipulation of bacteria (such as Escherichia coli) can be applied to developing recombinant strains of Z. mo — bilis and/or their metabolic engineering.

8. Integrant rec strains of Z. mobilis available for efficient ethanol production from glu­cose, xylose and arabinose. Ethanol concentrations above 60 g L-1 in 48 h reported for medium containing 65 g L-1 glucose, 65 g L-1 xylose.

9. Sequencing of ZM4 genome now provides information for its metabolic engineering for additional higher value products (e. g., succinic acid).

10. Potential for use of its enzymes for fine chemical biotransformations.

starch-based raw materials. Some of the reasons lie in the concerns that Z. mobilis may be less robust than yeast and more susceptible to contamina­tion in large-scale processes, as well as the lack of ethanol industry experience with large-scale bacterial fermentations. In addition, an established feed mar­ket exists for the high protein yeast by-product (as dried distiller’s grains) and any new market for a high protein by-product from a Zymomonas process would need to be established. The key issues and alternative capabilities are summarized in Table 4.

The construction of recombinant strains of Z. mobilis able to use the addi­tional C5 sugars xylose and arabinose have now opened up new opportunities as illustrated by the recently announced Dupont/Broin partnership to develop a Zymomonas-based process for conversion of corn stover to ethanol [43]. In an Integrated Corn Biorefinery (ICBR), this would be associated with conver­sion also of the corn starch to higher value products (e. g. to 1,3-propandiol using recombinant strains of Escherichia coli). Experience with large-scale re­combinant bacterial fermentations could provide a future platform as well for an increased range of higher value products generated via the metabolic en­gineering of micro-organisms such as Z. mobilis which are capable of both rapid and highly efficient sugar metabolism.

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