GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

14. TYPES OF FERMENTATION

14.3. LACTIC ACID FERMENTATION AND THE FAMILY LACTOBACILLACEAE

14.3.3. Heterolactic fermentation

Unlike Homofermentative lactic acid Bacteria, heterofermentative species lack key glycolytic Enzymes such as aldolase and Triosephosphate isomerase. The initial breakdown of glucose occurs exclusively via the phosphoketolase pathway (see Fig. 14.3). Ribulose-5-phosphate is converted into xylulose-5-phosphate by the action of epimerase. Subsequently, a phosphoketolase-catalyzed reaction cleaves xylulose-5-phosphate to yield glyceraldehyde-3-phosphate and acetyl phosphate. Glyceraldehyde-3-phosphate is metabolized to lactic acid via Pyruvate, while acetyl phosphate is reduced to ethanol through acetyl-CoA and acetaldehyde. This type of Fermentation is characteristic of Leuconostoc mesenteroides.

Other heterofermentative bacteria convert part or all of the acetyl phosphate into acetic acid, a process coupled with ATP synthesis. In this case, excess hydrogen is transferred to glucose, which is reduced to mannitol. Meanwhile, glyceraldehyde-3-phosphate is converted to lactate via pyruvate.

The fermentation of fructose proceeds According to the equation

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Here, fructose acts as an acceptor for excess reducing equivalents and is reduced to mannitol.

Heterolactic fermentation is carried out by Leuconostoc mesenteroides, L. fermentum, L. brevis, and Bifidobacterium bifidum.

Fermentation by Bifidobacterium bifidum. This bacterium is named for its characteristic V- and Y-shapes (Latin bifidus meaning cleft or branched). It is well known as the predominant microorganism in the gut of breastfed infants. All members of the genus Bifidobacterium are strict anaerobes and grow in an atmosphere containing 10% CO₂. Bifidobacteria catabolize glucose into lactic acid and acetate. Notably, they lack both aldolase and glucose-6-phosphate dehydrogenase. In these bacteria, glucose is converted into fructose-6-phosphate via The Glycolytic Pathway; fructose-6-phosphate is then cleaved by active phosphoketolases into acetyl phosphate and xylulose-5-phosphate. Xylulose-5-phosphate, in turn, yields acetyl phosphate and glyceraldehyde-3-phosphate. Acetyl phosphate is reduced to acetate, whereas glyceraldehyde-3-phosphate is converted to lactate via pyruvate.