Viruses are generally not only our enemy but also the enemy of many other organisms. Bacteriophages are viruses that infect specific bacteria. In this lesson, we'll discuss their basic structure and infection cycle.
Viruses Are Nonliving
Every year around the time the leaves start to turn brilliant colors of gold and red, you're reminded to grab a scarf, drink some hot apple cider and get your flu shot. The winter season is ahead, and with it comes a greater chance of catching the flu virus. There are great things about the winter, but the flu is not one of them.
Viruses affect not only humans but plants, animals, and even bacteria
We are all familiar with viruses as something that makes us ill. Countless viruses are all around us. They infect not only humans but also other organisms, such as animals, plants and bacteria. They don't all cause the flu, and most of them can't be treated or even vaccinated against.
So what exactly is a virus? A virus is an acellular, parasitic particle consisting of protein and nucleic acid. 'Acellular' refers to the fact that viruses are not made of cells. They can't reproduce on their own. They can't transport substances in and out of any membranes. They can't metabolize any sort of food. What can viruses do? Viruses can't really do anything on their own except invade a host cell. On their own, they do not exhibit characteristics of living things. For all these reasons, unlike the hosts that they infect, viruses are not alive.
There's huge diversity in viruses and what hosts they can infect. A bacteriophage is a virus that infects bacteria. Bacteriophages are nicknamed 'phages' for short. Though it's true that a bacteriophage can't really give a bacterium the flu, bacteriophages share some similar features with the types of viruses we routinely avoid. However, all viruses are also host-specific. Bacteriophages will infect a specific strain or species of bacteria but will not infect any other organism. They will not and cannot infect you. This is one of the reasons that bacteriophages are such a popular area of current research. In the growing epidemic of antibiotic-resistant bacteria, bacteriophage could potentially be used as a new type of treatment for bacterial infections. These viruses can recognize and kill human bacterial infections without destroying our own cells.
All viruses are surrounded by a capsid, or viral envelope made of proteins. This capsid can be made of different types of proteins and it can come in all different shapes, increasing the variety of these viruses. All viruses target a specific strain or species of bacteria and won't infect other species. Proteins in a viral capsid will recognize specific proteins or carbohydrates on the surface of the bacterial host cell wall that it's able to infect. All viruses also contain nucleic acid within this capsid. This nucleic acid can be double-stranded or single-stranded DNA or RNA.
Viruses cannot replicate on their own and attach to the bacterial cell wall of a host
Unlike most other viruses, bacteriophages have special tail fibers that aid in injecting the viral nucleic acid into the bacterial cell like a syringe. Because a virus can't replicate on its own, it requires a host. A bacteriophage recognizes a specific host and attaches to the bacterial cell wall. Proteins or carbohydrates on the outside of the bacteria serve as markers for bacteriophages to recognize and bind their targets. Next, using its tail fibers, it injects its nucleic acid into the bacterial host. At this point, the virus can enter one of two possible cycles.
The Lytic Cycle
The first is the lytic cycle. In the lytic cycle, bacteriophage will replicate and cause the bacterial cell to burst, or lyse, to release newly assembled phage. After injecting its nucleic acid into a bacterium, a phage will direct degradation of host DNA. In this way, it can ensure that its own nucleic acid is the only set of instructions in the host cell. All of the cell's resources can then be directed towards making more virus - not towards making more bacteria. In the next step, the bacteriophage will completely take over the cell machinery to replicate its nucleic acid, transcribe viral RNA and translate viral proteins. These new nucleic acid copies and viral proteins will be assembled into brand new bacteriophage within the cell. The last step of the lytic cycle is release of the viruses by lysing the bacterial cell wall so that the new viruses can go on to infect more hosts.
The steps of the lytic cycle
A bacteriophage may not be alive, but its success as a parasite has relied on some very life-like qualities. When it attaches to the cell and injects its nucleic acid, some bacteriophages must make a decision. Should it replicate and lyse the cell or stick around for a while? In the lytic cycle, the host cell obviously dies, and the viruses must find new hosts. This might be the best course of action if the bacterial host is under some kind of stress. If a bacterial host is thriving, however, a virus might decide to 'move in.' And why not? Imagine you had a friend that was an excellent host, providing good food, company, entertainment and a place to sleep. You might not want to leave either!
The Lysogenic Stage
When a host bacterial cell replicates its DNA, it replicates the virus and passes it on to new cells
If a bacterial host is growing and dividing, this gives a virus an opportunity to grow and divide within the cell, too. Here, the bacteriophage would attach to its host, inject its nucleic acid and then enter the lysogenic cycle. In the lysogenic cycle, phage nucleic acid will integrate into the bacterial genome. The term 'genic' in 'lysogenic' can remind you that the phage genes are incorporated into the bacterial genes, a step that does not occur in the lytic cycle. The prophage is the phage DNA that is integrated into the bacterial DNA. Every time the bacterial cell replicates its DNA and divides into two new cells, it will also replicate the viral genetic information and pass this on to new cells. The virus has now infected two hosts without any additional effort. This will continue to happen through multiple generations until the bacterial host is no longer hospitable. At this point, some bacteriophage are triggered to enter the lytic cycle.
In summary, we've learned that viruses are nonliving parasitic particles made of nucleic acid surrounded by a capsid made of protein. Bacteriophages are viruses that infect bacteria. After recognizing and attaching to a specific host, bacteriophages may replicate by the lytic cycle or the lysogenic cycle. In the lytic cycle, bacteriophage degrade the bacterial DNA and use bacterial resources to reproduce, eventually lysing the cell to release newly assembled bacteriophages. In the lysogenic cycle, bacteriophage nucleic acid integrates into the bacterial genome. This prophage, or phage DNA inserted into bacterial DNA, is then replicated with the bacterial DNA and passed down to dividing cells. The bacteriophage will lie dormant in the lysogenic cycle unless it is triggered to enter the lytic cycle, possibly by sensing that the bacterial cell is stressed and no longer the best host.
After watching this lesson, you should be able to:
- Define and describe the characteristics of bacteriophages
- Explain the steps of the lytic and lysogenic cycles