Today our topic is animal viruses. Viruses represent the very smallest forms of life, and they are among the smallest pathogens that can cause disease. The image on this slide is a transmission electron micrograph taken of the influenza virus. So this is the same virus that causes seasonal and pandemic flu. The colors that you see in the image are false. So they're added to the image just to make it easier to visualize the structures. It's a really gorgeous image. Viruses are tiny. So the fact that we're able to make an image like this amazes me every time I see it. Another amazing fact is that Louis Pasteur created a vaccine for rabies, which is caused by a virus, before we even knew what viruses were He'd been successful already creating a vaccine for anthrax. So what he did with rabies was he infected rabbits with the rabies virus. And then he sacrificed the animals. He dried out their spinal cords, and he used that material to save the life of a nine year old boy in 1885. It turns out we know now that drying out the spinal cords inactivated the virus. Rabies was not uncommon, and it's 100% fatal even now. So you can imagine that this vaccine was very well received. The discovery of viruses is attributed to a Russian microbiologist named Dimitri Ivanovsky, and he was exploring a mosaic disease of tobacco plants occurring in Eastern Europe. People were losing a lot of money to this particular disease. And so there was a lot of motivation to try to identify the cause. We now know that this is caused by the tobacco mosaic virus. And what Ivanovsky did was he cut up infected tobacco plants. He filtered them through a porcelain filter. He knew that the pores of these filters were tiny enough that bacteria would be trapped. And that's where he expected to find the pathogen causing the disease. He thought it would be a bacteria. But what he found instead was that the material that was filtered out was actually not infectious. It was actually the fluid that went through the filter that was infectious. And this was the first clue that for some infections, something smaller than bacteria, were responsible for the disease. So he showed that the cause of the tobacco disease was due to this filterable virus. And the term virus comes from the Latin meaning poison. This is a wonderful picture of Louis Pasteur in his own laboratory. It sure looks a lot different than our lab at CCSF. I'm always incredibly amazed at what these early microbiologists were able to discover with the limited tools that they had available to them. A microbiologist who specializes in the study of viruses is called a virologist. And among virologists, there is a big debate. So some virologists say that, yes, viruses are living and they can direct the processes of life. they're able to get into a cell and direct their own synthesis or copying. They say that you know, a virus is like a Xerox machine, it causes a cell to make millions of copies of itself. And they're clearly much more than inert, lifeless, molecules. So then you've got another group of virologists that says, nope, a virus can't multiply by itself. They need another life form to be copied. Humans don't need another life form to be copied in order to multiply. Right. We don't have a host organism that we have to get into. We can reproduce our own. Bacteria divide in half they don't need other organisms to divide. But viruses cannot divide on their own. They don't have all the machinery that a cell has, and they are unable to make copies of their proteins and nucleic acids like a cell can. Viruses actually have to infect a cell, take it over, and then use the cell as a manufacturing plant for making a new virus. So the people who argue that viruses are not living explained that they think of viruses as more of an infectious set of molecules that can't multiply on their own. We're also going to talk about prions in this lecture. Prions are just proteins. They're clearly infectious and they're definitely not living. So we're going to lump viruses and prions together since they both have this ambiguity about their classification as life. I think what you're going to see is that there is somewhat of a spectrum between things that are living and things that are clearly not living, and that maybe viruses fall into sort of a gray zone in between. One of the themes of this class is that bacteria that we interact with are mostly beneficial. The ones that are pathogenic are relatively rare. And this is also true of viruses in many ways. We know that viral infections can change the genetic makeup of a cell that's infected. And some of these changes are permanent and then are inherited when that cell divides. In sequencing the human genome, we find that about 8% of our DNA consists of remnants of ancient viruses, and another 40% is made up of repetitive DNA sequences that we also think came from viruses. So those are really big numbers. If you look at bacteria, they're about ten to 20% viral DNA So it's clear that viruses play a role in the changes in DNA over time. And we know that's an important part of the evolutionary process. This is one of my favorite cartoons. We know that bacteria can share DNA with other bacteria using a process called lateral or horizontal gene transfer. This is the sharing of DNA between individuals in the same generation. Now, humans can't do that. I can't just hand you some of my DNA if you wanted to have a specific trait that I have. All of the DNA you've gotten is from your parents. And this is called vertical gene transfer. But a virus can transfer gene between individuals. And that's evidenced by how much viral DNA is in our genomes. So this is a cute little comic, but it makes an important biological point, and that is that normally we think of humans as only being able to pass genes on to our offspring. However, because a virus can get into one person and sometimes grab a little bit of their DNA, when that virus is then copied and infects a new person, it can actually transfer some of that DNA to the new host. This is one of the ways that genetic information can be passed in the same generation between two individuals. This is also the basis for a lot of ideas behind gene therapy. Scientists are working on creating viruses with healthy genes that can replace disease genes. The idea is that you'd use a virus to infect the person who's sick, and the virus would then replace the disease gene with the healthy gene. We refer to viruses as obligate intracellular parasites. They don't independently fulfill the characteristics of life, as we discussed earlier. They require another cell, a host cell as a manufacturing plant for new virus. If you take the molecules in a virus, whether they're together or in pieces, they're completely inactive outside a host cell. Viruses are tiny. They're only about 20 to 450 nanometers. In diameter. That's much, much smaller than even a bacterial cell, which is on the order of one micron to ten microns. Human cells vary in size but average about 50 microns in diameter. The basic structure of all viruses includes genetic material, either DNA or RNA, but not both. Remember that prokaryotic and eukaryotic cells contain both DNA and RNA, so that's a big distinction between cells and viruses. Around the genetic material is a protein shell called a capsid. Many viruses also have an outer layer called an envelope, which is a lipid structure that's derived from the cell membrane of the host cell that was infected with the virus. And then all viruses have molecules on their surface, either on the capsid or on the envelope that determine what kinds of host cell they'll infect. Viruses don't infect every cell in your body. They infect specific cells. So for example, influenza viruses and coronaviruses infect the cells in your respiratory tract and those surface "spike" proteins or keys or what recognize those cells and allow the virus to get inside. By contrast, the hepatitis virus infects liver cells because it's spike proteins or keys will recognize liver cells. There are a lot of things that viruses don't have. Viruses lack enzymes for their metabolic processes. That's why they have to get inside of you. They don't have DNA polymerase. They don't have ribosomes to make proteins. Not all viruses have lipid envelopes, but those that do obtain them from the host cell, the cell that they infected So a virus needs the host cell manufacturing structures in order to make or synthesize their biological molecules and build new viruses from those molecules. Now, as I said, viruses are tiny. We said they were very small in comparison to a cell. They are so tiny that you can't see them with the light microscope, but you can use an electron microscope that does have enough resolution to allow us to visualize what a virus looks like. Just to give you a sense of how tiny viruses can be 50 million polio viruses can fit inside the average human cell. There's also a lot of size variation among viruses, and that's nicely illustrated in this picture. For comparison, here's a red blood cell. And now here is an E coli bacterium. And if you zoom in on the E coli, you can compare the size of a bacterium to some common viruses. All cells, no matter what type bacteria, archaea, and eukaryotes, they're all infected by different kinds of viruses. We often refer to the viruses that infect, bacteria as bacteriophages or phage Some phage have really complex structures. They look like a spaceship landing on a planet, but actually it's just a virus landing on a bacterial cell. OK, so that's our introduction to viruses. In our next set of slides We're going to do a general overview of structures that all viruses have in common.