Zymomonas mobilis Mutants with an Increased Rate of Alcohol Production

Two new derivatives of Zymomonas mobilis CP4 were isolated from enrichment cultures after 18 months of serial transfers. These new strains were selected for the ability to grow and produce ethanol rapidly on transfer into fresh broth containing ethanol and allyl alcohol. Ethanol production by these strains was examined in batch fermentations under three sets of conditions. Both new derivatives were found to be superior to the parent strain CP4 with respect to the speed and completeness of glucose conversion to ethanol. The best of these, strain YO2, produced 9.5% ethanol (by weight; 11.9% by volume) after 17.4 h compared with 31.8 h for the parent strain CP4. The addition of 1 mM magnesium sulfate improved ethanol production in all three strains. Two factors contributed to the decrease in fermentation time required by the mutants: more rapid growth with minimal lag on subculturing and the retention of higher rates of ethanol production as fermentation proceeded. Alcohol dehydrogenase isozymes were altered in both new strains and no longer catalyzed the oxidation of allyl alcohol into the toxic product acrolein. This loss of allyl alcohol-oxidizing capacity is proposed as a primary factor contributing to increased allyl alcohol resistance, although it is likely that other mutations affecting glycolysis also contribute to the improvement in ethanol production.