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Bacterial Fermentation Process & Products

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  • 0:01 Anaerobic Bacterial…
  • 1:13 Fermentation Basics
  • 2:37 Glycolysis is a Form…
  • 4:33 Other Fermentation Substrates
  • 5:07 Respiration vs. Fermentation
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Bacteria in anaerobic environments can break down organic compounds using fermentation. If you have ever eaten a fermented food, such as bread, yogurt or cheese, you have tasted the products of fermentation. Now, learn the details of this process.

Anaerobic Bacterial Metabolism

Bacteria must capture energy from food sources in order to grow and reproduce, or in other words, to stay alive. When we discuss bacteria capturing energy, what we are really talking about is the ability of bacterial cells to use a chemical substrate (a food) to generate ATP. ATP is the high-energy molecule used for storing energy and building all the necessary cellular components.

In anaerobic environments, bacteria can capture energy and store it in ATP using either anaerobic respiration or fermentation. This lesson will introduce fermentation. Fermentation happens in a wide range of environments, but in case you think you personally don't care about fermentation, just think of the last time you had a refreshing beer, a creamy yogurt, a fragrant cheese or a warm slice of bread. The process and products of fermentation by bacteria and yeast are responsible for the rich flavors and textures encountered in these foods.

Fermentation Basics

Fermentation is the anaerobic catabolism of a single chemical compound using a series of redox transformations with the goal of generating ATP by substrate-level phosphorylation. That definition sounds complicated, so let's break it down a little bit.

Catabolism just refers to the breakdown of a chemical compound. During fermentation, the breakdown occurs through a series of redox transformations, which is just a fancy term for electron donating and electron accepting reactions. All of the redox transformations that take place act on atoms found in the original substrate molecule. Some atoms will act as electron donors and some as electron acceptors. The electron shuffling requires a series of enzymes that accelerate the reaction and catalyze the production of various intermediate forms of the initial substrate.

Through these fermentation reactions, the energy present in the chemical bonds of the substrate can be captured to produce ATP by substrate-level phosphorylation. Substrate-level phosphorylation is the production of ATP by transferring high energy phosphate groups from an organic molecule to ADP.

Glycolysis is a Form of Fermentation

Let's look at the classic example of fermentation: glycolysis.

Glyco- means sugar and refers to glucose and -lysis means to split. So glycolysis is the splitting of glucose! We will call the first stage of glycolysis the prep stage. In this stage, four enzymes are used to prepare glucose for the rest of the pathway. Glucose is split into two molecules of glyceraldehyde-3-phosphate. In the prep stage, there are no redox transformations yet and no energy has been released or captured. In fact, energy is put in at this stage to transform the glucose.

In the second stage, this is where the important action happens. This is where the redox transformations take place that lead to generation of ATP. First, an enzyme adds a phosphate group to those two molecules of glyceraldehyde-3-phosphate from the prep stage. These phosphate groups are high energy, and the next enzymes in the pathway transfer the phosphate groups to ADP to make ATP. Now we are making some energy! The final result from this stage is ATP, the electron carrier NADH and pyruvate.

In the final stage of glycolysis, everything gets wrapped up and the redox reactions get balanced out. Various enzymes act on the pyruvate, using NADH as an electron donor to transform the pyruvate to various fermentation end products. These end products can be things like ethanol, carbon dioxide, hydrogen or lactate. For the cell, the goal of generating ATP and balancing NADH levels is accomplished. The fermentation end products are merely wastes that can be excreted from the cell. Unless of course you are a brewer, baker or cheese maker, in which case those end products lend flavor and texture to your food products!

Other Fermentation Substrates

Of course, we can't limit our discussion to just glucose fermentation! Bacteria are culinary connoisseurs and have palates that prefer substrates beyond just glucose. Many organisms ferment various sugars, but many also ferment amino acids, nucleotide precursors and aromatic compounds. There are even some specialists that ferment strange compounds that no other organisms are known to ferment. But in every case the basic steps, redox transformations of the substrate, generation and consumption of NADH and production of ATP, occur.

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