Microbiology, part 43: Acellular Infectious Agents - Viruses - Components & Shapes

Hi, I'm Cathy with Level Up RN. In this video, I'm going to begin my coverage of topics from the acellular infectious agents section of our microbiology flashcard deck. Specifically, I'll be talking about viruses, including key characteristics of viruses, virus components, and virus shapes. I've included a quiz at the end of the video to test your knowledge on some of the key facts I'll be covering. So definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards on viruses so you can follow along with me and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on in your micro class. Viruses are acellular pathogens. This means that they are not composed of cells and do not contain cells. Viruses require a host cell in order to survive. They basically hijack the host cell's machinery to reproduce, synthesize viral proteins, and assemble virus components. This makes them obligate intracellular parasites. Viruses come in a variety of sizes, but in general, they are very small, usually between 20 and 900 nanometers in length. Each virus has a host range, which is the range of species and cell types that the virus can infect.

So for example, a bacteriophage is a virus that only infects bacterial cells. They do not infect human cells. A virulent phage is a bacteriophage that infects a bacterial cell and causes the cell to lyse or burst, which results in cell death. Whereas a temperate phage integrates its genetic material into the host cell's chromosome, and that genetic material is replicated when the host cell replicates. So in the next video, in this video playlist, we will go into detail about the different ways that a bacteriophage can replicate. Of note, when you see the term virion, that is referring to a virus particle that is outside the host cell. So it's basically the extracellular form of a virus that is capable of infecting a new host. Let's now talk about the different components that make up a virus. So each virus will contain genetic material. This may be DNA or RNA, but not both. And then in addition to the DNA or RNA, each virus will have a capsid, which is a protein coat that surrounds and protects the genetic material. The capsid is made up of capsomeres, which are protein subunits that interlock to form the capsid.

Many viruses also have an envelope, which is a phospholipid membrane that surrounds the virus. This envelope is derived from the host cell when the virus pinches off from the host cell during a process called budding. A virus that lacks an envelope is referred to as a non-enveloped virus or a naked virus. Some viruses also have spikes, which are protein structures that extend outward and away from the virus. These spikes allow the virus to more effectively attach to the host cell, enter the cell, and cause infection. So viruses that have spikes tend to be more virulent. An example of a virus that has spikes is the coronavirus, which causes COVID-19. Now let's talk about key virus shapes, which include a helical shape, a polyhedral shape, and a complex shape. A virus with a helical shape has a cylindrical or rod-shaped capsid with the viral nucleic acid coiled up inside the capsid. An example of a virus with a helical shape is the tobacco mosaic virus. A virus with a polyhedral shape has a nucleic acid surrounded by a multi-sided capsid. An example of a virus with a polyhedral shape is the adenovirus. An icosahedron is a polyhedron with 20 equilateral triangular sides, which makes the virus appear spherical in shape.

An example of a virus with an icosahedral shape is the poliovirus. And of note, helical viruses and polyhedral viruses may also have an envelope. Lastly, some viruses will have a complex shape, which includes features of both helical and polyhedral viruses. The most common example of a virus with a complex shape is a bacteriophage. So as you can see in the image, the head of the bacteriophage has a polyhedral shape, whereas the tail of the bacteriophage has a helical shape.

All right, it's quiz time, and I have five questions for you.

Question number one, what do you call the protein coat that surrounds and protects the nucleic acid of a virus?

The answer is...the capsid.

Number two, what do you call the protein subunits that make up a capsid?

The answer is...capsomeres.

Number three, what is a naked virus?

The answer is...a virus that lacks an envelope.

Number four, what viral shape is shown here?

The answer is...helical.

And number five, what do you call a complete virus particle outside the host cell?

The answer is...a viria.

All right. That's it for this video. I hope you found it to be helpful. Take care and good luck with studying.