MICROBIAL FERMENTATION AND ITS TYPES

FERMENTATION

• Fermentation is a metabolic process that converts the sugar to acids and gases, or alcohol.
• Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product.
• French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes.
• The science of fermentation is known as Zymology.

1. ALCOHOLIC FERMENTATION

• Many fungi and some bacteria, algae, and protozoa ferment sugars to ethanol and CO₂ in a process called Alcoholic fermentation (Example Zymomonas mobilis and Saccharomyces cerevisiae).
• Pyruvate is decarboxylated to Acetaldehyde, which is then reduced to Ethanol by Alcohol dehydrogenase with NADH as the electron donor.
• Alcoholic fermentation by yeasts produces alcoholic beverages; CO₂ from this fermentation causes bread to rise.

2. LACTIC ACID FERMENTATION

• Lactic acid fermentation, the reduction of pyruvate to lactate, is even more common.
• It is present in bacteria (lactic acid bacteria, Bacillus), algae (Chlorella), some water molds, protozoa, and even in animal skeletal muscles.
• Lactic acid fermentation can spoil foods, but also is used to make yogurt, sauerkraut, and pickles.
• Lactic acid fermenters can be separated into two groups. They are (i) Homolactic fermenters and (ii) Heterolactic fermenters.

(i) Homolactic fermenters

• Homolactic fermenters use the Glycolytic pathway and directly reduce almost all their pyruvate to lactate with the enzyme lactate dehydrogenase.
• In Homolactic fermentation, the pyruvic acid formed in the EMP pathway is reduced by the enzyme lactate dehydrogenase to lactic acid. Here also 2 ATPs are generated for one glucose molecule.
• The bacteria Lactobacillus sp., Streptococcus sp. and Pediococcus sp. are involved in Homolactic fermentation. 

(ii) Heterolactic fermenters

• Heterolactic fermenters form substantial amounts of products other than lactate; many produce lactate, ethanol, and CO₂ by way of the Phosphoketolase pathway.
• The heterofermentative bacteria do not use Glycolysis, but use a Phosphoketolase pathway for fermentation.  Only one ATP is produced per molecule of glucose.
• The bacteria Lactobacillus brevis, Lactobacillus fermentum and Leuconostoc sp. are involved in Heterolactic fermentation.

3. FORMIC ACID FERMENTATION

• Many bacteria, especially members of the family Enterobacteriaceae, can metabolize Pyruvate to Formic acid and other products in a process sometimes called the Formic acid fermentation.
• Formic acid fermentations are very useful in identification of members of the Enterobacteriaceae.
• There are two types of Formic acid fermentation. They are (i) Mixed acid fermentation and (ii) Butanediol fermentation.

(i) Mixed acid fermentation

• Mixed acid fermentation results in the excretion of Ethanol and a complex mixture of acids, particularly Acetic, Lactic, Succinic and Formic acids.
• If formic hydrogenlyase is present, the formic acid will be degraded to H₂ and CO₂. This pattern is seen in Escherichia sp., Salmonella sp., Proteus sp. and other genera.

(ii) Butanediol fermentation

• Pyruvate is converted to Acetoin, which is then reduced to 2,3-butanediol with NADH.
• Butanediol fermentation is characteristic of Enterobacter sp., Serratia sp., Erwinia sp. and some species of Bacillus sp.

4. BUTYRIC ACID FERMENTATION

• The fermentation of Carbohydrates with the predominant formation of Butyric acid is causes by many Anaerobic bacteria belonging to the genus Clostridium, particularly Clostridium acetobutylicum.
• The first stage in the breakdown of Carbohydrate by Butyric acid fermentation are analogous to the corresponding stages in Alcoholic fermentation, up to the point of formation of Pyruvic acid from which Acetyl CoA is formed during Butyric acid fermentation.
• Acetyl CoA is the precursor for Butyric acid.

5. ACETONE – BUTYL ALCOHOL FERMENTATION

• The bacterium Clostridium acetobutylicum ferments carbohydrates, forming predominantly Butyl alcohol and Acetone.

6. PROPIONIC ACID FERMENTATION

• The bacteria Propionibacterium sp. coverts the Succinic acid to Propionic acid, Acetic acid and CO₂. This is called Propionic acid fermentation.

7. ACETIC ACID FERMENTATION

• Bacteria of the genus Acetobacter sp. oxidize Ethanol to Acetic acid. This is called Acetic acid fermentation.

8. GLUCONIC ACID FERMENTATION

• Certain mold fungi capable of oxidizing the aldehydes group of Glucose to Gluconic acid. This is called Gluconic acid fermentation.