Introduction to Bacteriophage Infections: Videos & Practice Problems

Bacteriophages, or phages, are viruses that specifically infect bacterial cells, facilitating horizontal gene transfer through transduction. Phage infections are categorized into productive infections, which can either cause cell lysis or not, and latent state infections, where the viral genome integrates into the host chromosome. Lytic phages lead to cell death through lysis, while lysogenic phages can remain dormant as prophages, potentially altering the host's genotype. Understanding these mechanisms is crucial for exploring phage therapy and bacterial resistance.

Map of Lesson on Bacteriophage Infections

In this video, we're going to begin our introduction to bacteriophage infections. First, we need to recall from our previous lesson videos that bacteriophages are also sometimes referred to as phages. These bacteriophages are viruses that specifically infect bacterial cells. Also recall that these bacteriophages can serve as a vehicle for horizontal gene transfer through transduction. These phage infections can actually occur in a variety of different ways. As we move forward in our course, we'll discuss the different methods in which bacteriophages can infect cells. Below, we have our map of the lesson on bacteriophage infection, which is the image you see. Since this is a map of our lesson, you can use it to guide you as we discuss phage infections further. We'll explore the leftmost branches of our map first and then zoom out to explore these branches in order from left to right.

Notice in this map that phage infections can be categorized as either productive infections that cause cell death, productive infections that do not cause cell death, or latent state infections. Regarding productive infections that cause cell death, these include lytic or virulent phages. As we proceed, we'll see that this type causes the host cell to lyse, rupturing and bursting, releasing the phages as the cell dies. Regarding latent state infections, these are caused by lysogenic phages, also known as temperate phages. Lysogenic phages integrate silently into the host chromosome and occasionally, they may cause something known as lysogenic conversion, which helps prevent superinfection. We will delve deeper into this topic later in our course.

Lastly, for productive infections that do not cause cell death, we have filamentous phages. These phages infect the host cell via the pili of the host cell. Below, you can observe a filamentous phage infecting the host cell through its pili. We will discuss more about these filamentous phages and the various types of phage infections as we move forward in our course. But for now, this concludes our brief introduction to bacteriophage infections. We'll explore each of these phages in more detail in their own separate videos as we progress. I'll see you all in our next video.

Productive vs. Latent State Infections

In this video, we're going to distinguish between productive infections and latent state infections. And so, really, there are 2 main types of phage infections. Once again, there are productive infections and there are latent state infections. Now the productive infections, as its name implies, new phage particles are going to be produced. And so the production of these new phage particles during a productive infection could potentially kill or lyse the cell when the cell ruptures, releasing viruses. Or on the other hand, a productive infection may not kill the host cell, and so it's possible the host cell does not die, but it will continuously release and produce viruses. And so regardless if this host cell dies or not, productive infections are going to produce new phage particles.

Now during a latent state infection, the viral genome is going to silently integrate into the host cell's chromosome. But during a latent state infection, there will not be the production of any new phage host chromosome, the integration of the viral genome can change the genotype of the host cell. And by changing the genotype, of course, that's going to be changing the set of genes that it has. And also the viral genome that has been integrated into the host cell's chromosome can replicate as the host cell multiplies and divides into more cells.

And so if we take a look at our image down below, notice the left-hand side of the image is focusing in on a productive infection. And so productive infections may or may not kill the host cell, but, what you'll notice is during productive infections new phage particles are going to be produced, And those phage particles, once again, it may kill or rupture or lyse the cell, releasing phage particles, or it may not kill or rupture the cell. But, new phage particles are going to be produced during a productive infection.

And so on the right over here, what we're focusing in on is a latent state infection. And during the latent state infection when the bacteriophage infects the bacteria, what you'll notice is that the phage genome is going to integrate into the host cell's chromosome. And so you can see the integrated phage DNA is being pointed out right here in this image. And so when the phage DNA is integrated like this, the cell is not going to be producing new phages. And so the host cell is going to basically host the viral genome, and the viral genome is going to be silently integrated into the host cell's chromosome.

And so, the integration of the viral genome here, once again, can change the genotype of the host cell, and so it could give the host cell new properties. But, the viral genome can also replicate as this host cell multiplies as well. And so, we will get to learn and talk more about productive infections and latent state infections as we move forward in our course. But for now, this here concludes our brief differentiation of productive and latent state infections. And once again, 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.

Some viruses, such as human herpesvirus 1, infect a cell without causing symptoms; these are called:

• A. Latent viruses.
• B. Phages.
• C. Lytic viruses.
• D. Slow viruses.

Answer: (A)

Lytic & Lysogenic Bacteriophages

In this video, we're going to differentiate between lytic and lysogenic bacteriophages. And so lytic phages are also sometimes referred to as virulent phages. And these are bacteriophages that can cause a productive infection. This productive infection specifically will kill the infected host cell by cell lysis. Now lysogenic phages, on the other hand, are sometimes referred to as temperate phages. These lysogenic or temperate phages are bacteriophages that can carry out a productive infection, or they could carry out a latent state infection. We'll be able to learn more about these lysogenic and temperate phages as we move along, but there are a few things that you should note here. The phage DNA of a lysogenic or temperate phage has the ability to integrate itself into the host chromosome, creating what scientists refer to as a prophage. And a prophage is really just the integrated phage DNA. This creates what is called a lysogen. A lysogen is a cell carrying a prophage. This prophage, the integrated phage DNA, can either remain integrated indefinitely for long periods of time, or the prophage can actually excise itself or remove itself from the host chromosome in order to begin a lytic or productive infection.

If we take a look at our image down below, we can get a better understanding of this. Notice at the top, we're showing you bacteriophage attachment, where a bacteriophage like this one here is able to attach itself to a bacterial cell, and the phage will inject its DNA into the host bacteria. If it is a lytic phage, then it will be able to conduct the lytic cycle. As the lytic cycle implies with the term lytic, it is going to be a productive infection where new viruses are going to be made, and it is going to result in cell lysis. You can see the lytic here in the lytic cycle is for the lysis that occurs, the cell rupturing. Notice that these bacteriophages are being produced, making it a productive infection, and it does indeed lyse and kill the cell, and so all the phages are able to be released through lysis. Now if it is a lysogenic or a temperate phage, then it can induce the lysogenic cycle. In the lysogenic cycle, what you'll notice is that the injected phage DNA has the ability to integrate itself into the bacterial chromosome to create a prophage. Once again, the prophage is really just the integrated phage DNA that has integrated itself into the host chromosome. Once the phage has integrated itself into the host chromosome, it can then be replicated alongside the host chromosome and be passed down to future progeny. It is possible for this prophage to excise itself, to remove itself, and to shift into a lytic cycle. That is what this green arrow that you see here over towards the lytic cycle represents that a phage that is lysogenic can integrate to create a prophage, and the cell that contains the prophage is called a lysogen, as we mentioned above in our text. This prophage, again, has the ability to excise itself, remove itself, and to initiate a lytic cycle. We'll be able to talk more about this idea here as we move forward in our course, but for now, this here concludes our brief introduction to lytic bacteriophages and lysogenic bacteriophages. And so I'll see you all in our next video.

Here’s what students ask on this topic:

What are bacteriophages and how do they infect bacterial cells?

Bacteriophages, or phages, are viruses that specifically infect bacterial cells. They attach to the bacterial cell surface and inject their genetic material into the host. This genetic material can either lead to a productive infection, where new phage particles are produced, or a latent state infection, where the viral genome integrates into the host chromosome. In productive infections, the host cell may lyse, releasing new phages, or it may continuously produce phages without dying. In latent state infections, the viral genome remains dormant within the host chromosome, potentially altering the host's genotype and being replicated along with the host's DNA.

What is the difference between lytic and lysogenic bacteriophages?

Lytic bacteriophages, also known as virulent phages, cause productive infections that result in the lysis and death of the host cell. They hijack the host's machinery to produce new phage particles, which are released when the cell bursts. Lysogenic bacteriophages, or temperate phages, can either enter a lytic cycle or integrate their DNA into the host's chromosome, becoming a prophage. In the lysogenic cycle, the prophage remains dormant and replicates with the host's DNA, potentially altering the host's genotype. The prophage can later excise itself and enter the lytic cycle, leading to cell lysis.

How do productive infections differ from latent state infections in bacteriophages?

Productive infections in bacteriophages result in the production of new phage particles. These infections can either cause the host cell to lyse, releasing the new phages, or allow the host cell to survive while continuously producing phages. In contrast, latent state infections involve the integration of the phage genome into the host's chromosome without producing new phages. The integrated viral genome, known as a prophage, can replicate with the host's DNA and remain dormant, potentially altering the host's genotype. The prophage can later excise itself and initiate a productive infection.

What is lysogenic conversion and how does it prevent superinfection?

Lysogenic conversion occurs when a lysogenic phage integrates its genome into the host's chromosome, potentially altering the host's genotype and phenotype. This integration can provide the host with new properties, such as resistance to certain environmental stresses or immunity to superinfection by other phages. Superinfection is the infection of a bacterium already carrying a prophage by another phage. The presence of the prophage can prevent superinfection by producing repressor proteins that inhibit the replication of incoming phages, thereby protecting the host cell from additional phage attacks.

What role do filamentous phages play in bacteriophage infections?

Filamentous phages are a type of bacteriophage that cause productive infections without killing the host cell. They infect the host cell via the pili and continuously produce new phage particles, which are released without causing cell lysis. This allows the host cell to survive and continue to function while producing phages. Filamentous phages are important in studying phage-host interactions and have potential applications in biotechnology and phage therapy, where they can be used to deliver genetic material to bacterial cells without causing cell death.