GENERAL MICROBIOLOGY - T.P. Pirog - 2004

14. TYPES OF FERMENTATION

14.2. ALCOHOLIC FERMENTATION

14.2.2. Ethanol Production by Bacteria

The pathway of ethanol formation known in Yeasts (Glycolysis, the Pyruvate decarboxylase reaction, and the reduction of acetaldehyde) has been found among all studied Bacteria exclusively in Sarcina ventriculi.

The bacterium Zymomonas mobilis degrades glucose via the Entner-Doudoroff Pathway; pyruvate is cleaved in the pyruvate decarboxylase reaction into acetaldehyde and CO₂, and acetaldehyde is subsequently reduced to ethanol (see Fig. 14.2). The sole Fermentation products in this case are ethanol, carbon dioxide, and minor amounts of lactic acid.

During fermentations carried out by certain species of enterobacteria and clostridia, ethanol serves as a byproduct. Here, the precursor of ethanol—acetaldehyde—is formed not from pyruvate, but via the reduction of acetyl-CoA.

Heterofermentative lactic acid bacteria such as Leuconostoc mesenteroides produce ethanol through an entirely different pathway (see Fig. 142). Glucose is degraded via the phosphoketolase pathway to pentose phosphate. Xylulose-5-phosphate is then converted into acetyl phosphate and glyceraldehyde-3-phosphate with the participation of phosphoketolase:

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Acetyl phosphate is reduced to ethanol by acetaldehyde dehydrogenase and Alcohol dehydrogenase. Another product of glucose Cleavage, glyceraldehyde-3-phosphate, is reduced to lactate via pyruvate.