There has recently been considerable interest in the redirection of metabolism in bacteria such as E. coli for the overproduction of specific metabolites and higher value products. At a commercial level, the large-scale production by Tate & Lyle/Dupont of 1,3-propandiol using a highly engineered strain of E. coli is indicative of an increasing trend towards such bio-based pro­cesses. The fast specific rates of sugar uptake by Z. mobilis, its highly efficient metabolism for a specific product (ethanol), and its relatively small genome size (facilitating genetic manipulation) may make it an ideal candidate for producing other metabolites via its genetic engineering.

As shown in Fig. 7, Z. mobilis has an incomplete TCA cycle and the po­tential exists via “knock out” mutation to redirect metabolism away from end-products such as lactate and ethanol, towards higher value products like succinic acid. As reported recently by Kim et al. [42], succinic acid overproducing Z. mobilis strains have been developed by disruption of the genes for pyruvate decarboxylase (pdc) and lactate dehydrogenase (ldh). Such strains can produce relatively high concentrations of succinic acid at yields of 1.73 mole/mole glucose (86% theoretical). The yield was reported to be more than 30% greater when compared to those of other succinic acid — producing bacteria such as Actinobacillus succinogenes and Mannheimia suc- ciniciproducens (about 1.34 mole/mole glucose). These strains of Z. mobilis were also reported to exhibit higher overall rates of succinic acid production (1.62 gL-1 h-1) under Na bicarbonate supplemented conditions compared to those of other succinic acid producing bacteria (1.35 gL-1 h-1).

Entner-Doudoroff Pathway

f

I

2-keto-3-deoxy-6-P-gluconate

4.3