Your gut bacteria on fermentation: a matter of survival


60 second summary

  • Fermentation is the process by which bacteria release energy
  • Its it is important in allowing the recycling of molecules to restart the biological cycle for energy release
  • Fermented food tastes sour because of the lactic acid produced by bacteria in fermentation

Fermentation- a word we commonly hear associated with yoghurt, beer, sauerkraut and more. But what does it actually mean? Fermentation is a type of biological mechanism called respiration, which you’ll be an expert on after reading the below article.

What is respiration?

Respiration- the release of energy in living organisms

Overall, this may be classified into to main categories:

  • Aerobic respiration= with oxygen
  • Anaerobic respiration and fermentation= without oxygen.

Some cells are unable to respire using oxygen because they do not have the correct genes or enzymes required . An example or a anaerobic bacterium is E.coli.

Some organisms have evolved the adaptation that if environmental conditions change and oxygen does become available, they may switch on genes allowing them to respire aerobically .[1] These are called facultative organisms.

ATP- the energy currency in cells

In cells some ATP is produced via a mechanism called glycolysis, which is a fancy word for the breaking down of the sugar glucose.

If oxygen is present, aerobic respiration can occur. If it is not either fermentation or anaerobic respiration occurs.

The main difference between the two is that the fermentation produces lactic acid ( which is what causes cramps during exercise! )whilst anaerobic respiration’s final product is a molecule called pyruvate.

Glycolysis can continue again and again in an endless cycle even without any oxygen. But how can this be?

Surely all organisms need oxygen to survive? The answer is that required molecules are regenerated, essentially a recycling system. Many biological systems work on opposites to balance reactions out.

The science– In glycolysis a molecule called NAD+ is reduced forming NADH. Later the same molecule is oxidised reforming NAD+ [2].

Oxidation= loss of electrons

Reduction= gain of electrons

They are opposites!

Energy production in respiration with vs without oxygen

Respiration without oxygen releases 2 molecules of ATP per glucose molecules. This may seem reasonable but in the presence of oxygen 38 ATP’s per glucose can be formed, making aerobic respiration much more efficient.

Fermented foods

One of the reasons why many fermented foods taste sour is because of lactic acid produced as a by product of fermentation.

The science: The equation which summarises the process, is as follows:

Pyruvate  +  NADH ↔ lactic acid  + NAD+ [3]

Note that the through fermentation the NAD+, a substrate in the initial step of glucose breakdown mentioned above, is reformed!

Common lactic acid producting bacteria include Lactobacillus Leuconostoc and Streptococcus.

Types of fermentation

Overall there are 2 types of fermentation.

  1. Homolactic fermentation- where the only product is lactic acid. Commonly used in yoghurt production
  2. Heterolactic fermentation- where lactic acid, ethanol and carbon dioxide is produced. Seen in fermentation of vegetables like cucumbers into pickles.

The particular flavour of each fermented food is determined by the types of other organic acids produced in side reactions.

Overall, fermentation is an essential pathway for keeping bacteria alive as it allows the regeneration of molecules for respiration, the release of energy. Humans take advantage of this process by fermenting foods which portrays its benefits as an ancient preservation process as well as taste enhancer and a superb probiotic!

[1] Openstax. (n.d.). Microbiology. [online] Available at: [Accessed 10 Jul. 2019]. [2] Thoughtco. (2019). The Difference Between Fermentation and Anaerobic Respiration. Available at: [Accessed 10 Jul. 2019].

[3] Pink Monkey. (n.d.). Fermentation. [online] Available at: [Accessed 10 Jul. 2019].

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