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• Defined as an energy yielding process
• Organic molecules serve as both electron
donors and electron accepters.
• The molecule being metabolized does not
have all its potential energy extracted from it.
(not completely oxidized.)
• NAD+ is almost always reduced to NADH
• Remember that metabolism involves the
oxidation of the substrate. These electrons are
removed from the organic molecule and most
often given to NAD. (This is true both in
fermentation and respiration).
• Fermentation results in a excess of NADH
• Accumulation of NADH causes a problem
for anaerobes. They have too much of it
and it prevents further oxidation of
substrate due to a lack of an NAD+ pool to
accept electrons.
• In many fermentation pathways, the steps
after energy generation are performed in
part to get rid of the NADH:
pyruvate + NADH + H+ → lactate + NAD+
Glycolysis - Embden-Meyerhoff-Parnas
pathway (EMP)
• The most commonly used series of reactions for
oxidizing glucose to pyruvate.
• EMP is so ubiquitous. However, it is not the only
method for the fermentation of glucose.
• It is an essential part of many organisms
• A complete fermentation pathway begins
with a substrate, includes glycolysis and
results in various end-products.
• The different fermentation pathways
typically are named for the end products
that are formed.
• yeast, convert NADH back to NAD+ in a
process called ethanol fermentation. In this
process, the pyruvate is converted first to
acetaldehyde and carbon dioxide, then to
• Pyruvate is often an important intermediate
• Many of the reactions that we will look at
eventually end up making pyruvate.
• Pyruvate is a valuable intermediate because it
can be used for cell synthesis.
• Many different enzymes can act on it. It gives
the microbe flexibility.
Fermentation Vs Aerobic
C6H12O6------------>2C2H5OH + 2CO2 (2ATP) ∆G = 33 Kcal ‫تخمیر‬
C6H12O6 + 6O2------------>6CO2 + 6H2O (36ATP) ∆G =
‫تنفس‬686 Kcal_
Incomplete oxidation of glucose or other
carbohydrates in the absence of oxygen
Uses organic compounds as terminal electron
Yields a small amount of ATP
Production of ethyl alcohol by yeasts acting on
Formation of acid, gas & other products by the action
of various bacteria on pyruvic acid
(EMP pathway)
Entner–Doudoroff pathway
• The Entner–Doudoroff pathway describes an
alternate series of reactions that catabolize
glucose to pyruvate using a set of enzymes
different from those used in either glycolysis
or the pentose phosphate pathway
Homolactic Fermentation
(Lactic acid)
- It is the sole end product.
- Pathway of the homolactic acid bacteria
- The bacteria are used to ferment milk and milk
products in the manufacture of yogurt,
buttermilk, sour cream, cottage cheese, cheddar
Mixed Acid Fermentations.
• Mainly the pathway of the Enterobacteriaceae.
• End products are a mixture of lactic acid,
acetic acid, formic acid, succinate and ethanol.
• Possibility of gas formation (CO2 and H2) if the
bacterium possesses the enzyme formate
dehydrogenase, which cleaves formate to the
Butanediol Fermentation
• Mixed acids and gases but in addition, 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.
• 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
Butyric acid fermentations
(Butanol-acetone fermentation)
• Butyric acid fermentations as well as the butanolacetone fermentation, 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 and acetone were discovered as the main
end products of fermentation by Clostridium
Propionic acid fermentation.
• Carried out by the propionic acid bacteria which include
corynebacteria, Propionibacterium and Bifidobacterium.
• Propionic acid bacteria ferment lactate to acetic acid, CO2
and propionic acid.
• The formation of propionate is a complex and indirect
process involving 5 or 6 reactions.
• 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.
Fermentations in bacteria that proceed through the Embden-Meyerhof
pathway. Representive bacteria that utilize these pathways are in
shown in BLUE.