Virus Discovery – Viruses

by Vincent Racaniello, PhD

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    00:00 a mottling of the leaf and as a consequence you can't sell the leaf to make tobacco. This virus was first identified in 1892, by a scientist called Ivanovsky. What he was trying to do was figure out what was causing this disease of plants. So what he would do is to take diseased leaves and grind them up in a broth and then he would place them through a filter and this filter is called the Chamberland filter, where you can either use vacuum or pressure to push a fluid through a porcelain filter with very, very small holes in it, and those holes were about 0.2 microns in diameter. It was known that these kinds of filters would retain bacteria, so Ivanovsky was trying to prove that this disease was caused by a bacterium. But what he found was that the material that went through the filter, if he took that and applied it to plant leaves, it would cause an infection. So it was either smaller than any bacteria known to date or it was something different. A few years later Beijerinck reproduced Ivanovsky's studies with tobacco mosaic disease, he called it a Contagium Vivum Fluidum and finally the name virus was applied to this. This is a Latin term meaning slimy liquid or poison. They didn't quite understand what they were dealing with at the time, but they thought it was something new. About the end of the 1800s, 1898, the first animal virus was discovered, the agent of foot-and-mouth disease, an important disease of cattle and they found that this agent was also filterable through a 0.2 micron filter. So the key concept arising here, not only is the agent very small, it goes through a 0.2 micron filter, but it only grows in the host, it will not grow in a broth.

    01:45 If you put these filtrates into a broth, they will not grow. They will only grow if you put them in a host. Now the key here is a 0.2 micron filter and this remained part of the definition of a virus for many years until recently when we found these giant viruses that are much bigger that I just told you about.

    02:05 So virus discovery after these initial years proceeded very quickly. In 1901 the first human virus was discovered, yellow fever virus. Rabies virus followed in 1903. Variola virus or smallpox virus 1906. Poliovirus in 1908, a virus that's about to be eradicated. Rous sarcoma virus, one of the first cancer-causing viruses in 1911. And those viruses we've talked about a bit that infect bacteria, bacteriophages discovered in 1915, and not until 1933 was influenza virus discovered. In the 1940s, for the first time, scientists were able to see what a virus looked like. Up until that point they weren't sure if it were just a liquid or chemical, but in the 1940s the electron microscope was invented and for the first time we could see what a virus looked like. So we could see that they were actually particles.

    03:07 And here in panel A is an electron micrograph of a bacteriophage, the virus that infects bacteria, a really interesting virus with a spherical head and a long tail. Here in panel B is tobacco mosaic virus, a very long rod shaped particle, the first virus discovered that causes a disease of plants. These are the particles of rabies virus, they were found to be bullet shaped and finally spherical virus particles. Now this virus is called rotavirus, it causes a diarrheal illness in people. Viruses have a variety of shapes, but the key point about this experiment, for the first time we realize they were particles.

    03:52 Today we know incredible details about viruses, we can solve their three-dimensional structures, that is, we know where every atom is located in three dimensions. We can reconstruct images like this one. We even know the chemical formulas for a virus. Here is the chemical formula for poliovirus, you can see it's pretty long, that may be the biggest chemical formula you've ever seen, but we can do this for any virus. We know amazing details about all of them.

    About the Lecture

    The lecture Virus Discovery – Viruses by Vincent Racaniello, PhD is from the course Microbiology: Introduction.

    Included Quiz Questions

    1. By transmitting a disease to tobacco plants using a cell-free filtrate of diseased leaves.
    2. Pasteur showed that viruses could replicate in a sterile medium.
    3. Leeuwenhoek saw viruses in his microscope.
    4. Robert Koch showed that viruses grown in broth could cause disease.
    1. 0.2 microns
    2. 20 microns
    3. 22 microns
    4. 2 microns
    5. 2.2 microns

    Author of lecture Virus Discovery – Viruses

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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