Mixed fermentations

Butanediol Fermentation. Forms mixed acids and gases as above, but, in addition, 2,3 butanediol from the condensation of 2 pyruvate. The use of the pathway decreases acid formation (butanediol is neutral) and causes the formation of a distinctive intermediate, acetoin. Water microbiologists have specific tests to detect low acid and acetoin in order to distinguish non fecal enteric bacteria (butanediol formers, such as Klebsiella and Enterobacter) from fecal enterics (mixed acid fermenters, such as E. coli, Salmonella and Shigella).

Butyric acid fermentations, as well as the butanol-acetone fermentation (below), are run by the clostridia, the masters of fermentation. In addition to butyric acid, the clostridia form acetic acid, CO₂ and H₂ from the fermentation of sugars. Small amounts of ethanol and isopropanol may also be formed.

Butanol-acetone fermentation. Butanol and acetone were discovered as the main end products of fermentation by Clostridium acetobutylicum during the World War I. This discovery solved a critical problem of explosives manufacture (acetone is required in the manufacture gunpowder) and is said to have affected the outcome of the War. Acetone was distilled from the fermentation liquor of Clostridium acetobutylicum, which worked out pretty good if you were on our side, because organic chemists hadn't figured out how to synthesize it chemically. You can't run a war without gunpowder, at least you couldn't in those days.

Propionic acid fermentation . This is an unusual fermentation carried out by the propionic acid bacteria which include corynebacteria, Propionibacterium and Bifidobacterium . Although sugars can be fermented straight through to propionate, propionic acid bacteria will ferment lactate (the end product of lactic acid fermentation) to acetic acid, CO₂ and propionic acid. The formation of propionate is a complex and indirect process involving 5 or 6 reactions. Overall, 3 moles of lactate are converted to 2 moles of propionate + 1 mole of acetate + 1 mole of CO₂, and 1 mole of ATP is squeezed out in the process. The propionic acid bacteria are used in the manufacture of Swiss cheese, which is distinguished by the distinct flavor of propionate and acetate, and holes caused by entrapment of CO₂.

 

REFERENCES:

Text Books:

1. Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth Edition). Jones and Bartlett Student edition.

2. Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson - Microbiology: An Introduction. Benjamin Cummings.

Reference Books:

1. Lansing M. Prescott, John P. Harley and Donald A. Klein. Microbiology. Mc Graw Hill companies.