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History of Viruses

The term virus was coined by the Dutch microbiologist Martinus Beijerinck, who showed, using methods based on the work of Ivanovski that tobacco mosaic disease is caused by something smaller than a bacterium. He coined the Latin phrase "contagium vivum fluidum" (which means "soluble living germ") as the first idea of the virus.

Viral diseases such as rabies, yellow fever and smallpox have affected humans for centuries.

In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught cowpox and was found to be immune to smallpox, a similar, but devastating viral diseases. Edward Jenner developed the smallpox vaccine based on these findings. In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire, observed local women inoculating their children against smallpox.

After lengthy vaccination campaigns, the World Health Organization (WHO) certified the eradication of smallpox in 1980.

In the late 19th century, Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.

Dimitri Ivanovski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. He passed crushed leaf extracts of infected tobacco plants through the filter, and then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitude smaller than bacteria.

In the early 20th century, Frederick Twort discovered that bacteria could be infected by viruses. Felix d'Herelle, working independently, showed that a preparation of viruses caused areas of cellular death on thin cell cultures spread on agar. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of electron microscopy provided the first look at viruses. In 1935, Wendell Stanley crystallized the tobacco mosaic virus and found it to be mostly protein. A short time later, the virus was separated into protein and nucleic acid parts

Types of Viruses Based on their Symmetry

  • Icosahedral Viruses: Human adenovirus
  • Enveloped Viruses: HIV
  • Helical Virus: Tobacco Mosaic Virus (TMV)
  • Complex Viruses: Cow pox virus

Nucleic Acid

A virus may employ either DNA or RNA as the nucleic acid. Rarely do they contain both; however cytomegalovirus is an exception to this, possessing a DNA core with several mRNA segments. By far most viruses have RNA. Plant viruses tend to have single-stranded RNA and bacteriophages tend to have double-stranded DNA. Some virus species possess abnormal nucleotides, such as hydroxymethylcytosine instead of cytosine, as a normal part of their genome.

Tobacco Mosaic Virus


T4 Bacteriophage




Virus without protein coat is called viroids. Viroids have a free single stranded RNA with low molecular weight.



Lichens are symbiotic associations of a fungus (the mycobiont) with a photosynthetic partner/ algae (the photobiont also known as the phycobiont) that can produce food for the lichen from sunlight.



Phycobiont or Photobiont


The photobiont is usually either blue- green alga or cyanobacterium. Some lichens contain both green algae and cyanobacteria as photobionts; in these cases, the cyanobacteria symbiont component may specialize in fixing atmospheric nitrogen for metabolic use.

In the natural environment, lichen "provides" the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its substrate. As for the alga, it uses the minerals and water to make food for the fungus and itself.

Lichens are named based on the fungal component, which plays the primary role in determining the lichen's form. The fungus typically comprises the majority of lichen's bulk, though in filamentous and gelatinous lichens this is not always the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens.



A major eco-physiological advantage of lichens is that they are poikilohydric (poikilo - variable, hydric - relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation.

Lichens may be eaten by some animals, such as reindeer, living in arctic regions.

Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants. Hence, they have been widely used as pollution indicator organisms.

Some of examples for lichens are following:
crustose (paint-like, flat), e.g., Caloplaca flavescens  filamentous (hair-like), e.g., Ephebe lanata  foliose (leafy), e.g., Hypogymnia physodes  fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis

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