Bacteriophage: Lysogenic Phage Infections: Videos & Practice Problems

Lysogenic phages, or temperate phages, can integrate their DNA into a host's chromosome, forming a prophage. This process, known as lysogeny, allows the prophage to replicate with the host cell, creating lysogens that carry the integrated phage DNA. Lysogenic conversion can alter the host's phenotype, changing surface structures and providing immunity to superinfection. In some cases, it can also confer disease-causing properties, such as toxin synthesis. Understanding these mechanisms is crucial for grasping bacterial resistance and pathogenicity.

Bacteriophage: Lysogenic Phage Infections

In this video, we're going to continue to talk about bacteriophages by focusing specifically on lysogenic phage infections. Recall from our previous lesson videos that lysogenic phages are also sometimes referred to as temperate phages, and these phages may not immediately produce new phages. After lysogenic phage DNA enters into a host cell, one of 2 possible scenarios are going to happen. The first scenario is that after injection of the lysogenic phage DNA, it could lead to the production of new phages by the lytic cycle, as we already discussed in our previous lesson videos. But the second possible scenario of the lysogenic phage DNA is that it could lead to integration of the phage DNA into the host chromosome, forming what's known as a prophage. Recall that a prophage is just the phage DNA that has been integrated. This occurs by the lysogenic cycle. The lysogenic cycle or lysogeny is the lysogenic phage replicating where the phage DNA remains silent or inactive in the cell. New phages are not being produced. However, the prophage, the integrated phage DNA can replicate inside of the lysogen. Recall that the lysogen is just the bacterial cell that carries the prophage, the integrated phage DNA. The prophage, which is within the chromosome of the host cell, can be replicated as the host cell replicates. The prophage can be passed down to future generations of the bacteria through the normal replication processes.

If we take a look at our image down below, we can get a better understanding of the lysogenic cycle as it occurs in a specific type of bacteriophage known as the LambdaPhage. At the very top we're showing you bacteriophage attachment and genome entry. Then the bacteriophage injects its genome into the phage. If this is a temperate or a lysogenic phage, then the phage could lead directly to the lytic cycle as we talked about previously. However, it could also go into the lysogenic cycle. In the lysogenic cycle, integration would be the 3rd step after genome entry. In integration, what happens is the phage DNA is going to integrate into the host cell's chromosome forming a prophage. The prophage is really just the integrated phage DNA, and the bacterial cell along with the prophage is referred to as a lysogen. This is where the lysogenic cycle gets its name because a lysogen is a bacterial cell that contains a prophage. As the lysogen begins to replicate and divide, it will also replicate and divide the prophage. Notice that the prophage here is also being replicated and passed down, and the prophage replicates with the host chromosome, with the lysogen's chromosome. All of the cells that result from replication are going to contain the prophage as well. The viral DNA is being replicated and passed down to the future generations of cells. All offspring are going to end up being lysogens because they all contain the prophage. Under the right conditions, the prophage is able to excise itself, leading us to the 5th step, excision. Excision is really just the removal of the phage DNA. The excised phage DNA, when this prophage is able to excise or remove itself, and the prophage can be excised by a viral enzyme, that can allow the lytic cycle to begin. This means that the lysogenic cycle has the ability to transition back into a lytic cycle, and it can ultimately cause the cell to lyse and lead to a productive infection. However, the lysogenic infection here, notice that there are no viruses being produced in this period. Only the prophage is being replicated, the integrated phage DNA. We'll be able to get some practice applying these concepts as we move forward in our course. But for now, this here concludes our brief introduction to the lysogenic phage infection. I'll see you all in our next video.

Which steps of the lytic and lysogenic cycles are the same?

• A. Attachment & genome entry.
• B. Replication & synthesis.
• C. Integration & excision.
• D. Assembly & release.

Answer: (A)

Some bacterial viruses (lysogenic phages) carry viral DNA that acts like an episome. When a bacterial cell that is infected by a lysogenic phage replicates, what happens to the viral DNA?

• A. The viral DNA remains separate from the bacterial chromosome and is not passed on to the daughter cells.
• B. The viral DNA is incorporated into the bacterial chromosome and passed on to the daughter cells.
• C. That viral DNA remains separate from the bacterial chromosome but is still passed on to the daughter cells.
• D. The viral DNA is degraded by the bacterial cell.

Answer: (B)

Lysogenic Conversion

In this video, we're going to introduce lysogenic conversion. Lysogenic conversion is really just a phenotypic change of a lysogen based on the prophage that it carries. Once again, recall from our previous lesson videos that a prophage is really just the integrated phage DNA, and the lysogen is the cell that contains the prophage. Lysogenic conversion occurs when the lysogen changes its phenotype due to the integrated phage DNA, the prophage. Typically, lysogenic conversion can result in the change of the host cell surface structures. These host cell surface structures are what the phages rely on to attach and initiate an infection. By changing the host cell surface structures, the phages will not be able to attach. Thus, cells can become immune to what is known as superinfection, which is really just infection by the same type of phage. When lysogenic conversion changes the host cell surface structures, the same phage will not be able to infect the cell, making the lysogen immune to superinfection.

Occasionally, lysogenic conversion can also give the cell disease-causing properties. For example, the synthesis of a toxin, the ability to synthesize a toxin. Let's take a look at our image down below to get a better understanding of lysogenic conversion. On the left, we're showing you a bacterial cell, and before a viral infection, we have a bacterial cell with a normal phenotype. However, notice that when a bacteriophage virus comes and infects the cell and injects its genome, that genome occasionally can insert itself into the chromosome to create a prophage, which is what we're showing you here. The prophage is the integrated phage DNA. Notice that the surface receptors of this lysogen are capable of being converted through lysogenic conversion to alter the phenotype. Over here, we have an altered phenotype where now the surface structures are being altered through the prophage. The alteration of these surface structures makes it so that the same phage is not able to bind and induce an infection. This lysogen with an altered phenotype due to lysogenic conversion is going to be immune to a superinfection or immune to infection by the same phage. In addition to superinfections, sometimes lysogenic conversion can give these lysogens disease-causing properties, like, for example, the ability to synthesize a specific type of toxin.

This here concludes our brief introduction to lysogenic conversion, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

A strain of Corynebacterium diphtheriae has been infected by the Phage-beta, and the phage genome has been integrated into the host chromosome. Thus, this Corynebacterium diphtheriae is ______ and the integrated phage genome is a _______.

• A. Lytic, lysogen.
• B. Lysogen, prophage.
• C. Lytic cell, temperate phage.
• D. Lysen, prophage.
• E. Lysogenic, lysogen.
• F. None of the answers are correct.

Answer: (B)

Here’s what students ask on this topic:

What is the difference between the lytic and lysogenic cycles in bacteriophages?

The lytic and lysogenic cycles are two different pathways that bacteriophages can take after infecting a host cell. In the lytic cycle, the phage DNA takes over the host cell's machinery to produce new phages, eventually causing the host cell to lyse and release the new phages. In contrast, the lysogenic cycle involves the integration of phage DNA into the host cell's chromosome, forming a prophage. This prophage replicates along with the host cell's DNA without causing immediate harm. Under certain conditions, the prophage can excise itself and enter the lytic cycle, leading to the production of new phages and cell lysis.

What is lysogenic conversion and how does it affect bacterial cells?

Lysogenic conversion is a process where a lysogen (a bacterial cell carrying a prophage) undergoes a phenotypic change due to the integrated phage DNA. This can result in alterations to the host cell's surface structures, making it immune to superinfection by the same type of phage. Additionally, lysogenic conversion can sometimes confer disease-causing properties to the bacterial cell, such as the ability to synthesize toxins. These changes can significantly impact bacterial resistance and pathogenicity, making lysogenic conversion an important concept in microbiology.

How does a prophage integrate into the host cell's chromosome?

After a lysogenic phage injects its DNA into a host cell, the phage DNA can integrate into the host cell's chromosome through a process called integration. This involves the phage DNA recombining with the host DNA, forming a prophage. The prophage remains dormant within the host's chromosome and replicates along with the host cell's DNA during cell division. This allows the prophage to be passed down to future generations of the bacterial cell, maintaining the lysogenic state until conditions trigger its excision and entry into the lytic cycle.

What triggers the transition from the lysogenic cycle to the lytic cycle in bacteriophages?

The transition from the lysogenic cycle to the lytic cycle in bacteriophages can be triggered by various environmental factors and stress conditions. These triggers can include UV radiation, chemical mutagens, or other stressors that damage the host cell's DNA. When such conditions arise, the prophage excises itself from the host chromosome through the action of viral enzymes. Once excised, the phage DNA initiates the lytic cycle, leading to the production of new phages and eventual lysis of the host cell.

What is a lysogen and how does it differ from a regular bacterial cell?

A lysogen is a bacterial cell that carries a prophage, which is the integrated phage DNA within its chromosome. Unlike a regular bacterial cell, a lysogen has the potential to undergo lysogenic conversion, leading to phenotypic changes such as altered surface structures and immunity to superinfection by the same phage. Additionally, lysogens can sometimes acquire disease-causing properties, such as the ability to produce toxins. These characteristics make lysogens distinct from regular bacterial cells that do not carry prophages.