Prototrophs vs. Auxotrophs - Online Tutor, Practice Problems & Exam Prep

Prototrophs are original parent strains with functional genes that synthesize their own growth factors, represented by a plus sign (e.g., His⁺). In contrast, auxotrophs are mutants with DNA mutations that prevent them from synthesizing essential growth factors, requiring these factors from their environment, indicated by a minus sign (e.g., His^-). Understanding these differences is crucial for studying microbial growth and metabolism, particularly in genetic research and biotechnology applications.

Prototrophs vs. Auxotrophs

This video, we're going to begin our lesson on prototrophs versus auxotrophs. And so first we need to understand that some cell mutations can actually alter their growth factor requirements, or what growth factors need to be present in order for them to grow and survive. These mutations that alter the growth factor requirements of cells lead us to these two groups, the prototrophs and the auxotrophs. Now, the prototrophs are commonly represented with a plus sign as you see here.

The prototrophs represent the original parent strains that have no mutations, and therefore, because they have no mutations, they do not require any additional growth factors. The root proto which is found in the word prototroph is a root that means the original form. Once again, these prototrophs are the original parent strains, they have no mutations, and therefore they do not require additional growth factors. The reason that they do not require additional growth factors is that these prototrophs have functional genes that allow them to synthesize or build their own growth factors from other molecules. These growth factors do not need to be in the immediate environment.

The auxotrophs, on the other hand, are commonly represented with a minus sign as you see here. Unlike the prototrophs, these are mutants that contain mutations in the DNA. They contain specific mutations that require the auxotrophs to have additional growth factors. The root auxo, which is found in the word auxotrope is a root that means increase, and this is referring to the increased additional growth factors that are required. These specific mutations that the auxotrophs have actually prevent them from synthesizing their own growth factors from other molecules. Therefore, the growth factors that they need must be provided and available in the environment for them to grow and survive. If they are not available, the auxotrophs will not be able to survive.

A memory tool to help you remember: the plus sign represents prototrophs. Use the "p's" here to remember that prototrophs are represented with the plus sign. The minus sign, as you see here, is going to represent the auxotrophs. In our example below, we're going to look at Histidine Prototrophs or hist+ cells since, again, the plus represents prototrophs, and we're also going to look at histidine auxotrophs or His- cells, where again the minus sign represents the auxotrophs. Histidine is one of the 20 amino acids used to build proteins. It is required for cells to build their proteins, which is why histidine is the growth factor of interest.

Prototrophs, like what we see over here on the left, are again the original parent strain with no mutation and they do not require these additional growth factors. Notice that the prototroph is represented as a His+ bacteria because the plus represents the prototrophs. His+ just means it has a normal His+ gene that does not have any mutations. This normal His+ gene allows the prototroph to synthesize its own growth factor. As long as the prototroph has this normal His+ gene, it can make its own histidine and that histidine does not have to be available in the environment.

The auxotroph is going to have increased growth factor requirements. In this case, the auxotroph is going to require histidine to grow because it is a mutant His- bacteria where again the minus sign represents the auxotroph. This mutant His- bacteria has a mutation in the His gene, and we call this a mutant His- gene. Because it has that mutation, it prevents the synthesis of its own growth factor and prevents the synthesis of its own histidine. Therefore, histidine must be available in the environment for the auxotroph to grow. If histidine is not available in the environment for the auxotroph, then the auxotroph is going to die. This cell requires histidine to grow, whereas the prototroph does not require histidine to grow because it is capable of synthesizing its own histidine from other molecules.

This concludes our brief introduction to prototrophs and auxotrophs. We'll be able to cover an example problem and more practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Prototrophs vs. Auxotrophs Example 1

So here we have an example problem that's asking if His⁺ cells can survive on an agar plate that does not contain the amino acid histidine. To answer this problem, we first need to recognize that His⁺ cells refer to prototrophs. We know this because the plus sign represents prototrophs. Prototrophs are the original parent strain that has no mutation and therefore does not require additional growth factors. Hence, since this is a prototroph, it will not require the addition of histidine. The correct answer is: yes, His⁺ cells can survive on an agar plate without histidine because they do not need histidine to grow and survive. This is because the His⁺ prototrophs are capable of synthesizing their own histidine from other molecules. Again, the correct answer for this problem is answer option a, and b is not correct. If, however, the problem were to state His^- instead of His⁺, then we would be discussing auxotrophs. The His^- auxotrophs would require histidine to be available; the correct answer would be option b. But as it stands, this problem is about His⁺, so the correct answer is answer option a. That concludes this example problem, and I'll see you all in our next video.

A mutant strain of bacteria requiring additional growth factors that is not seen in the wildtype strain is called:

• A. An autotroph.
• B. A heterotroph.
• C. A prototroph.
• D. An auxotroph.
• E. A phototroph.

Which of the following answers about His+ prototrophs and His- auxotrophs is incorrect?

• A. His+ prototrophs are the parent bacterial cells with no mutations.
• B. His+ prototrophs require the amino acid histidine to grow.
• C. His- auxotrophs are the mutant bacterial cells which require growth factors to grow.
• D. His- auxotrophs cannot grow without the amino acid histidine.
• E. All of the above answers are correct.

Here’s what students ask on this topic:

What is the difference between prototrophs and auxotrophs?

Prototrophs are the original parent strains of microorganisms that have no mutations in their DNA, allowing them to synthesize their own growth factors from other molecules. They are represented by a plus sign (e.g., His+). Auxotrophs, on the other hand, are mutants with specific DNA mutations that prevent them from synthesizing essential growth factors. As a result, auxotrophs require these growth factors to be present in their environment to survive and grow. They are represented by a minus sign (e.g., His-). Understanding these differences is crucial for studying microbial growth, metabolism, and genetic research.

Why do auxotrophs require additional growth factors?

Auxotrophs require additional growth factors because they have specific mutations in their DNA that prevent them from synthesizing these essential compounds on their own. These mutations disrupt the normal metabolic pathways that would otherwise allow the organism to produce the necessary growth factors from other molecules. As a result, auxotrophs must obtain these growth factors from their environment to survive and grow. If the required growth factors are not available in the environment, the auxotrophs will not be able to grow and will eventually die.

How are prototrophs and auxotrophs represented in genetic notation?

In genetic notation, prototrophs are represented by a plus sign (e.g., His+), indicating that they have functional genes and can synthesize their own growth factors. Auxotrophs are represented by a minus sign (e.g., His-), indicating that they have mutations in their genes that prevent them from synthesizing essential growth factors, requiring these factors to be present in their environment for growth and survival.

What role do prototrophs and auxotrophs play in genetic research?

Prototrophs and auxotrophs play a crucial role in genetic research by serving as models to study gene function, mutation effects, and metabolic pathways. Prototrophs, with their ability to synthesize growth factors, provide a baseline for normal cellular function. Auxotrophs, with their specific mutations, help researchers understand the consequences of genetic changes and identify the genes involved in synthesizing essential compounds. This knowledge is valuable for developing genetic engineering techniques, studying microbial metabolism, and creating strains with desired traits for biotechnology applications.

Can a prototroph become an auxotroph? If so, how?

Yes, a prototroph can become an auxotroph through mutations in its DNA. If a mutation occurs in a gene responsible for synthesizing a specific growth factor, the prototroph loses its ability to produce that growth factor. As a result, it becomes an auxotroph, requiring the growth factor to be present in its environment for survival and growth. This process highlights the importance of genetic stability and the impact of mutations on cellular function.