Bacteriophage: Lytic Phage Infections - Online Tutor, Practice Problems & Exam Prep

The lytic cycle of bacteriophages involves five key steps: attachment, genome entry, synthesis, assembly, and release. Initially, the phage attaches to specific receptors on the bacterial cell. It then injects its DNA, while the protein coat remains outside. Phage enzymes degrade the host's DNA during synthesis, allowing for the production of new phage components. These components are assembled inside the host, leading to cell lysis, which releases new phages to infect neighboring cells. This cycle is crucial for understanding viral replication and host interactions in microbiology.

Bacteriophage: Lytic Phage Infections

This video, we're going to continue to talk more about bacteriophages by focusing specifically on the lytic phage infections. And so recall from our previous lesson videos that the lytic cycle is a productive infection, meaning that it allows for the production of new phages, allowing for lytic phage reproduction and increasing the number of lytic phages. Now as its name implies, the lytic cycle ends up resulting in the lysis of the host cell, which will kill the host cell and release the newly produced phages. Now the lytic cycle actually occurs in a series of 5 steps that we have numbered down below in our image. And so if we take a look at our image down below, we can get a better understanding of this lytic cycle.

And so in the very first step of the lytic cycle, what we have is attachment. And in this step, the phage is going to attach to very specific receptors on the host bacterium's cell envelope. And so what you'll notice is here we have our bacteriophage and down below we have our bacterial cell. And inside the bacterial cell here, we have the bacterial chromosome. And what you'll notice is that the very first step of the lytic cycle is just for this bacteriophage to attach to the surface of the host bacterium by interacting with very specific receptors on the host bacterium.

Then in step number 2 of the lytic cycle, what we have is genome entry. And so this is when the phage, the bacteriophage, is going to inject its genome, or its DNA for example, into the host bacteria. However, it is important to note that the protein coat of the bacteriophage is not injected. It remains on the outside of the host cell. And so what you'll notice here in this image is we have our bacteriophage here, and the protein coat of the bacteriophage remains on the outside of the host cell. However, the genome is going to enter into the host cell. And so the DNA here in this case, the phage DNA, is being injected into the host cell.

Now in step number 3 of the lytic cycle, we have synthesis. And so what happens is phage enzymes are going to be synthesized, and those phage enzymes are going to degrade the host DNA. And so, notice that the bacterial chromosome here, in the synthesis stage, is being degraded and chopped up into a bunch of small pieces, which essentially is going to inactivate the bacterial host chromosome. And in addition to that, viral genome and viral proteins are going to be made. And so, the phage proteins and genomes are going to be produced. They're going to be made in this synthesis step.

Then in the 4th step, what we have is assembly. And in this assembly step, what happens is all of the phage components that were made, including the phage proteins and the phage genomes are going to be assembled inside the bacterium to create more phages. And so what you'll notice is that all of these different pieces of the phage are going to be assembled together to put together the entire phage and this occurs inside of the bacterial cell.

And then finally in the 5th and final step of the lytic cycle, we have release. And as its name implies, this is when the host cell is going to lyse. And so the host cell lyses and as it lyses, it releases all of the newly infectious bacteriophages that were produced. And so you can see here the lysis of our bacterial cell is occurring, the rupturing of the cell, and all of these assembled bacteriophages are being released, and they can go on to start a brand new lytic cycle infection on another neighboring bacterial host cell.

And so really this here concludes our brief lesson on lytic phage infections and the 5 steps of this lytic infection cycle. And we'll be able to get some practice applying these concepts and learn more as we move forward in our course. So I'll see you all in our next video.

Which is NOT a step in a lytic phage reproductive cycle?

• A. Packaging of the phage's cytoplasm.
• B. Recognition and attachment to the host cell.
• C. Phage gene expression.
• D. Assembly of new phages.
• E. Entry of the phage genome.

Which of the follow statements about the bacteriophage lytic cycle is false?

• A. The lytic cycle is a productive infection which results in the host cell's death.
• B. The lytic cycle results in the host cell lysing and new bacteriophages emerging.
• C. In the lytic cycle the host cell's 'machinery' is used to create new viral components.
• D. In the lytic cycle the viral genome is incorporated into the host's genome.

Which of the following statements about the lysogenic cycle of a bacteriophage is true?

• A. A bacteriophage in the lysogenic cycle will kill the host cell by causing cell lysis.
• B. A bacteriophage in the lysogenic cycle can transition into the lytic cycle.
• C. A bacteriophage in the lysogenic cycle is a productive form of viral infection.
• D. A bacteriophage in the lysogenic cycle degrades the host cell's genome.

Here’s what students ask on this topic:

What are the steps involved in the lytic cycle of bacteriophages?

The lytic cycle of bacteriophages involves five key steps: attachment, genome entry, synthesis, assembly, and release. In the attachment step, the phage attaches to specific receptors on the bacterial cell. During genome entry, the phage injects its DNA into the host cell, leaving the protein coat outside. In the synthesis step, phage enzymes degrade the host's DNA, allowing for the production of new phage components. These components are then assembled inside the host cell during the assembly step. Finally, in the release step, the host cell lyses, releasing new phages to infect neighboring cells. This cycle is crucial for understanding viral replication and host interactions in microbiology.

How does a bacteriophage attach to a bacterial cell during the lytic cycle?

During the lytic cycle, a bacteriophage attaches to a bacterial cell by binding to specific receptors on the bacterial cell envelope. This attachment is highly specific, as the phage recognizes and binds to particular molecules on the surface of the host bacterium. This specificity ensures that the phage infects only certain types of bacteria. The attachment step is the first crucial step in the lytic cycle, setting the stage for the subsequent injection of the phage genome into the host cell.

What happens during the genome entry step of the lytic cycle?

During the genome entry step of the lytic cycle, the bacteriophage injects its genetic material, typically DNA, into the host bacterial cell. The protein coat of the phage remains outside the host cell. This injection process is facilitated by the phage's tail structure, which punctures the bacterial cell envelope, allowing the phage DNA to enter the host cell. Once inside, the phage DNA can take over the host's cellular machinery to begin the synthesis of new phage components.

What is the role of phage enzymes during the synthesis step of the lytic cycle?

During the synthesis step of the lytic cycle, phage enzymes play a critical role in degrading the host bacterial DNA. This degradation inactivates the host's genetic material, preventing it from interfering with the production of new phage components. Additionally, the phage enzymes facilitate the synthesis of viral genomes and proteins, which are necessary for assembling new phages. By hijacking the host's cellular machinery, the phage ensures the efficient production of its own components, leading to the eventual assembly of new infectious phages.

How are new phages assembled during the lytic cycle?

During the assembly step of the lytic cycle, newly synthesized phage components, including viral genomes and proteins, are put together inside the host bacterial cell. This process involves the precise assembly of various phage parts, such as the head, tail, and tail fibers, to form complete, functional phages. The assembly occurs in a highly organized manner, ensuring that each new phage is correctly constructed and capable of infecting other bacterial cells. Once assembled, these new phages are ready for release during the final step of the lytic cycle.

What occurs during the release step of the lytic cycle?

During the release step of the lytic cycle, the host bacterial cell undergoes lysis, or rupture, which releases the newly assembled phages into the surrounding environment. This lysis is typically caused by phage-encoded enzymes that break down the bacterial cell wall. As the cell lyses, it releases numerous new phages, which can then go on to infect neighboring bacterial cells, initiating new lytic cycles. This release step is crucial for the propagation of the phage population and the continuation of the infection process.