Fermentation pathway is characterized by its essential trait, where an organic compound (e.g., pyruvic acid) is the final electron acceptor for the electrons removed from substrates of this pathway. Furthermore, this final acceptor is directly produced in the pathway; thus, no external acceptor is required.
As the result, by contrast to aerobic respiration, fermentation doesn’t need molecular oxygen as common electron acceptor. When accomplished, it produces a variety of low-energy compounds, termed end products; the nature of these products strongly depends on the species of bacteria.
Different species of bacteria can utilize an extremely broad number of organic substances to obtain energy and reducing power by means of various metabolic pathways.
Glycolysis (Gr. glycos – “sugar” and Gr. lysis – “dissolution”) is the most common pathway for degrading sugars. It is often termed as Embden-Meyerhoff pathway, named after the two scientists who identified its major steps in the 1930s.
Many bacteria and yeasts use this pathway to degrade glucose and other sugars. Here one glucose molecule is converted into two molecules of pyruvic acid, as well as into reducing power equivalents such as hydrogen atoms.
Pyruvic acid, generated from a primary set of reactions, is used further for both fermentation or respiration.
The differences in molecular energy between glucose and pyruvic acid are accumulated in high-energy bonds in ATP. ATP synthesis results from substrate phosphorylation, where energy-rich phosphate anhydride bond is directly transferred from organic donor molecules to ADP. The final ATP yield in fermentation is equal to 2 ATP molecules.
In the absence of respiration or photosynthesis, mirobial cells are completely dependent on substrate phosphorylation to gain energy. Therefore, in this case ATP synthesis ensues from chemical transformations of primary organic substances. A great variety of substrates are metabolized within diverse fermentation pathways.
For instance, a lot of bacteria may perform the lactic acid fermentation. Many of them produce only a single product (lactic acid) being called as homofermenters. Nonetheless, some other lactic acid-producing bacteria are heterofermenters, releasing CO2 and ethanol, as well as lactic acid end products.
Closely related species of bacteria can be reliably discriminated by their products of fermentation.
Due to the fermentation activity of acetic acid bacteria (Acetobacter spp.) acetic acid is formed (acetic acid fermentation).
Ethanol or alcoholic fermentation takes place under the influence of the enzymes of yeasts Saccharomyces cerevisiae (yeasts), mucor moulds, Zymomonas mobilis bacteria, etc.
Lactic acid fermentation is caused by the fermentative activity of Lactobacillus casei, Lactococcus lactis, etc. The enzymes of lactic acid bacteria break down glucose with the production of lactic acid. The representatives from family Enterobacteriaceae are heterofermenters and produce lactic, succinic, and acetic acids, ethanol, carbon dioxide and hydrogen.
Butyric acid fermentation is performed with the anaerobes Clostridium perfringens or Clostridium butyricum resulting in the production of butyric acid
Propionic acid fermentation is demonstrated by anaerobes from genus Propionibacterium spp.
Other Pathways of Glucose Degradation
The major scheme for the degradation of glucose harnesses the glycolytic pathway. However, not all bacteria are able to use this process. They derive energy through other pathways.
The alternate route is termed the Entner-Doudoroff pathway or 2-keto-3-deoxyphosphogluconate pathway. It gives pyruvic acid from glucose oxidation via 2-keto-3-deoxyphosphogluconic acid intermediate. This pathway generates only 1 molecule of ATP after transformation of 1 molecule of glucose as substrate.
Another sugar metabolizing pathway that is present in most bacteria is termed the pentose phosphate pathway. The pathway is not of great significance for the production of energy, but it is valuable as the products of the pathway are 5-carbon and 4-carbon molecules that serve as precursor metabolites for nucleic acid and amino acid synthesis. Further, it provides reducing power required in the biosynthesis of cell components. This pathway is important in the organisms that carry out fermentations where reducing power (NADH) is not available for biosynthetic reactions.
Both of these pathways as well as glycolysis, can operate in the presence or absence of oxygen. Nevertheless, Entner-Doudoroff pathway is most likely found in aerobic bacteria, e.g. Pseudomonas aeruginosa