Engineering lactic acid bacteria with pyruvate decarboxylase and alcohol dehydrogenase genes for ethanol production from Zymomonas mobilis

Lactic acid bacteria are candidates for engineered production of ethanol from biomass because they are food-grade microorganisms that can, in many cases, metabolize a variety of sugars and grow under harsh conditions. In an effort to divert fermentation from production of lactic acid to ethanol, plasmids were constructed to express pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH), encoded by the pdc and adhB genes of Zymomonas mobilis, in lactic acid bacteria. Several strains were transformed with the plasmids. and transcription of pdc and adhB was confirmed by northern hybridization analysis of transformants. PDC and ADH enzyme activities were at least 5- to 10-fold lower in these bacteria compared to Escherichia coli transformed with the same plasmid. Glucose fermentations were carried out, and some, but not all, of the transformed strains produced more ethanol than the untransformed parent strains. However, lactic acid was the primary fermentation product formed by all of the transformants, indicating that ADH and PDC activities were insufficient to divert significant carbon flow towards ethanol.