Viruses

Continuum

I have been fascinated from the detailed data of a typical encyclopedia, as Wikipedia, since microorganisms, virology, and microbiology were always a preferred subjects during my course of Biology.

For this reason, I would like to spend more time on viruses to provide to readers as well to myself data that look of a certain interest considering the period we are living and our interest for viruses in general.

History of Viruses

Louis Pasteur, a French biologist, and microbiologist was responsible of the discovery of the principle of vaccination, microbial fermentation, and pasteurization.

He is remembered for his breakthroughs in the causes and prevention of diseases and his discoveries have saved many lives since then. He reduced mortality from puerperal fever and created the first vaccines for rabies and anthrax.

His medical discoveries provided direct support for the germ theory of disease and its application in clinical medicine. He is best known as the discoverer of pasteurization, the technique of treating milk and wine to stop bacterial contamination.

Louis Pasteur is regarded as one of the three main founders of bacteriology and recognized as “Father of Microbiology”. He was responsible for disproving the doctrine of spontaneous generation performing experiments that showed that, without contamination, microorganisms could not develop. 

Here is how the discovery of the existence of particles like viruses happened thanks to Pasteur’s discoveries and experiments. Pasteur was unable to find a causative agent for rabies and was guessing about a pathogen too small to be detected by microscopes. In 1884, the French microbiologist Chamberland invented the Chamberland Filter with pores small enough to remove all bacteria from a solution.

In 1892, the Russian biologist Dimitri Ivanovsky used this filter to study what is now known as the Tobacco Mosaic Virus, he crushed leaf extracts from infected tobacco plants which remained infectious even after filtration to remove bacteria. He suggested the infection might be caused by a toxin produced by bacteria.

At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium, this was part of the “germ theory disease”. In 1898, the microbiologist Martinus Beijerinck repeated the experiments and became persuaded that the filtered solution contained a new form of infectious agent.

Beijerinck observed that the agent multiplied only in cells that were dividing; he called contagium vivum fluidum (fluid living germ) and re-introduced the word virus. He thought that viruses were liquid in nature, but later another scientist proved they were instead particulate. 

In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages, and a French-Canadian microbiologist  described as viruses that would produce areas of dead bacteria, when added to bacteria on an agar-plate.

He diluted a suspension of these viruses and discovered that the highest dilutions, or lower concentration, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor he could calculate the number of viruses in the original suspension.

Phages were proposed as a potential treatment for diseases such as typhoid and cholera, but then forgotten with the development of penicillin. The development of bacteria resistance to antibiotics has recently renewed interest in the therapeutic use of bacteriophages.

By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to pass filters, and their requirement for living hosts. Viruses had been grown only in plants and animals. In 1906, a scientist invented a method for growing tissues in lymph, and later more animal tissues and cells were experimented. Their method was not adopted until the 1950s when poliovirus was grown on a large scale for vaccine production.

Another breakthrough came in 1931, when two American pathologists grew influenza and different other viruses in fertilized chicken eggs. In 1949 poliovirus were grown in cultured cells from aborted human embryonic tissue. This work allowed the production of an effective polio vaccine.

The first images of viruses were obtained thanks to the invention of electron microscope in 1931 by German engineers, in 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it was mostly made of protein.

The tobacco mosaic virus was the first to be crystallized and its structure could be this way descripted in detail with the X-diffraction technique. On the basis of the X-ray crystallographic pictures, Rosalind Franklin discovered the full structure of the virus in 1955. 

In the same year was showed that purified tobacco mosaic virus RNA and its protein coat can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the one through which viruses were created within their host cells.

The second half of the 20th century was the golden age of virus discovery and most of the documented species of animal, plant, and bacterial viruses were discovered during these years.

In 1983 Luc Montagnier’s team at Pasteur Institute in France, isolated the retrovirus now called HIV. In 1989 a team at Chiron corporation discovered Hepatitis C virus.

To be continued.

Thanks For Reading

Mariarosaria M.

Wikipedia Rearrangement

Picture by Salon.com

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