Viruses
In 1892 Dmitri Ivanovski passed sap from sick tobacco plants through a porcelain filter that stopped all bacteria, and the sap could still make plants sick.
Something smaller than a bacterium
Tobacco mosaic disease makes tobacco leaves blotchy. In 1892 the Russian botanist Dmitri Ivanovski tried to find its cause using a porcelain filter that Charles Chamberland and Louis Pasteur had invented in Paris in 1884. Its pores are about 0.1 µm, small enough to remove all bacteria of 0.2 µm or more. To everyone’s surprise, the filtered sap still caused the disease. Ivanovski thought it must be an extremely small bacterium or a spore. In 1898 Martinus Beijerinck concluded that the agent was not a bacterium at all, and the word virus, Latin for poison, came into use. Ivanovski is credited as the original discoverer of viruses.
Nobody could see a virus until the electron microscope. In 1935 Wendell Stanley crystallized tobacco mosaic virus and initially described it as a protein. Bawden and Pirie later established that it contained both RNA and protein, reporting their work in 1936 and 1937. Today we know a virus particle, or virion, can be from 20 nm across for small viruses up to about 900 nm for large ones, much smaller than most cells.
Are viruses alive?
Viruses are acellular, not made of cells, so they are not placed on the tree of life. They carry genes, but only one kind of nucleic acid, either DNA or RNA and never both, wrapped in a protein coat called a capsid. Some also have an envelope, a small piece of membrane taken from the last host cell as the virus budded out. Many have protein spikes that let them attach to and enter a cell. A virus has no cytoplasm and lacks the genes for much of what it needs, so it can reproduce only inside a living host cell. That is why viruses are called obligate intracellular parasites.
Capsids come in three broad shapes. Helical capsids are rods, as in tobacco mosaic virus. Polyhedral capsids have many flat faces. Complex capsids combine parts, like the bacteriophages that infect bacteria: a many-sided head holding the genes, a tail and tail fibres that grip the bacterial surface.
Because a virus cannot copy itself, every new virus is made by a cell that has been taken over. In outline the steps are the same for most viruses. The virus attaches to a cell, using its spikes or tail fibres to grip particular molecules on the surface. Its genes get inside. The cell’s own machinery then copies the viral genome and builds capsid proteins from the viral instructions. New virions assemble and leave, either by bursting the cell or by budding out through its membrane, which is how enveloped viruses pick up their envelope.
Every kind of life has its viruses
Viruses infect plants, animals, fungi, protists, bacteria and archaea. Most can infect only one or a few species, and often only certain cell types within them; this is called the host range. Viruses that infect bacteria are called bacteriophages, or phages. Each kind of phage kills only certain bacteria, and that makes them useful: in 2006 the US Food and Drug Administration approved a spray of six phages that kills Listeria bacteria on lunch meats.
Even smaller than viruses are prions, about one-tenth the size of a typical virus. They contain no genetic material at all; they are abnormal proteins. Like viruses they are acellular, so they sit outside the tree of life too.
- 1884: Chamberland and Pasteur invent the porcelain filter.
- 1892: Ivanovski: the tobacco disease agent passes the filter.
- 1898: Beijerinck: the agent is not a bacterium.
- 1935: Stanley crystallizes tobacco mosaic virus.
- 2006: FDA approves a phage spray against Listeria on lunch meats.
| Capsid | Protein coat around a virus’s nucleic acid. |
| Envelope | Membrane some viruses take from the host cell. |
| Virion | A single virus particle. |
| Bacteriophage | A virus that infects bacteria. |
| Host range | The species and cell types a virus can infect. |
| Prion | Infectious abnormal protein with no genetic material. |
- Ivanovski 1892: the agent passed a bacteria-proof filter; Beijerinck 1898: not a bacterium.
- Viruses are acellular: DNA or RNA in a capsid, sometimes an envelope.
- They reproduce only inside host cells.
- Helical, polyhedral and complex capsids; 20 to 900 nm.
- What did Ivanovski’s 1892 experiment show?
- The cause of tobacco mosaic disease passed through a filter that stopped bacteria
- Bacteria cause tobacco mosaic disease
- Viruses are made of cells
- Plants cannot get diseases
- The Chamberland filter has pores of about 0.1 µm. How many nanometres is that?
- What does the word “virus” mean in Latin?
- Poison
- Small animal
- Seed
- Life
- A virus contains…
- either DNA or RNA, never both
- both DNA and RNA
- neither DNA nor RNA
- a nucleus
- Why are viruses called obligate intracellular parasites?
- They can reproduce only inside a living host cell
- They live only in water
- They eat cells from outside
- They are always harmless
- What is a capsid?
- The protein coat around a virus’s genes
- A bacterial cell wall
- A fungal filament
- A kind of ribosome
- Tobacco mosaic virus has which capsid shape?
- Helical
- Polyhedral
- Complex
- Spherical envelope only
- In what year did Wendell Stanley crystallize tobacco mosaic virus?
- A virus that infects bacteria is called…
- a bacteriophage
- a prion
- a protozoan
- a capsid
- Virions range from about 20 nm to about 900 nm. How many times larger is the largest than the smallest?
Sources for this course (15)
- OpenStax Microbiology 1.1 What Our Ancestors Knew (fermented drink from rice, honey and fruit in Neolithic China by 7000 BC, Saccharomyces cerevisiae in bread, Leeuwenhoek 1675 "animalcules" in rain water, Golden Age of Microbiology 1857 to 1914; openstax.org/books/microbiology/pages/1-1-what-our-ancestors-knew)
- 1.3 Types of Microorganisms (about 100 µm to see with the eye, animal cell about 10 µm, bacteria about 1 µm, viruses about 100 nm, three domains, coccus/bacillus/spirillum, archaea without peptidoglycan and in extreme places such as Morning Glory pool, protists, algae, diatoms 2 to 200 µm, protozoa with cilia, flagella or pseudopods, fungi with chitin walls, more than 1000 yeast species, molds and penicillin, helminths, prions about one-tenth of a virus; .../1-3-types-of-microorganisms)
- 3.1 Spontaneous Generation (Aristotle, van Helmont's mice recipe of 3 weeks, Redi 1668 with six jars, Needham 1745, Spallanzani, Pasteur 1858 gun-cotton filter, swan-neck flasks, Alhumbert Prize 1862, 1864 lecture; .../3-1-spontaneous-generation)
- 3.2 Foundations of Modern Cell Theory (Semmelweis 1847, 10 to 20 percent vs 1 percent, chlorinated lime; Snow; Pasteur 1856 fermentation; Lister 1867 carbolic acid; Koch's postulates; .../3-2-foundations-of-modern-cell-theory)
- 6.1 Viruses (acellular, obligate intracellular parasites, DNA or RNA never both, capsid, envelope, spikes, helical/polyhedral/complex, 20 nm to 900 nm, Ivanovski 1892 with the Chamberland filter of 1884, pore 0.1 µm, Beijerinck 1898 and virus = poison, Stanley 1935, host range, bacteriophages, FDA 2006 phage spray on lunch meats; .../6-1-viruses)
- 9.1 How Microbes Grow (binary fission, FtsZ Z ring, generation time, E. coli 20 minutes, human generation 25 years, M. tuberculosis 15 to 20 hours, M. leprae 14 days, 2^48 = 281,474,976,710,656 after 24 h at 30 minutes, Nn = N0 2^n, lag/log/stationary/death, budding, fragmentation, Epulopiscium; .../9-1-how-microbes-grow)
- Royal Society: Philosophical Transactions 12:821-831, published 25 March 1677, Leeuwenhoek's Dutch letter of 9 October 1676 "concerning little animals" (doi.org/10.1098/rstl.1677.0003)
- Royal Society archive EL/L1/22 (letter to Oldenburg dated at Delft, 18 pages, partial translation by Oldenburg)
- Royal Society blog "Celebrating Leeuwenhoek", 2023 (first letter via de Graaf, Phil. Trans. 19 May 1673; 190 letters in fifty years; died 26 August 1723 aged 90)
- N. Lane 2015, Phil. Trans. R. Soc. B 370:20140344 (upwards of 1,000,000 creatures in a drop of pepper-water; letter read aloud over several sessions)
- Lens on Leeuwenhoek, letter L-040 (Oldenburg printed less than half; Hooke succeeded later in 1677)
- CDC MMWR 53(34):783, 2004, 150th Anniversary of John Snow and the Pump Handle (31 August 1854, handle removed 8 September 1854, Vibrio cholerae isolated 1883; cdc.gov/mmwr/preview/mmwrhtml/mm5334a1.htm)
- CDC History of Smallpox and Smallpox timeline (Jenner 1796; 33rd World Health Assembly declared the world free of smallpox on 8 May 1980; cdc.gov/smallpox/about/history.html)
- American Chemical Society, Discovery and Development of Penicillin landmark (Fleming 1928 at St Mary's Hospital London, paper June 1929, Florey and Chain at Oxford from 1939; acs.org/education/whatischemistry/landmarks/flemingpenicillin.html)
- Fleming 1929, Br J Exp Pathol 10:226. Sender, Fuchs and Milo 2016, PLoS Biol 14:e1002533 (3.8 × 10^13 bacteria and 3.0 × 10^13 human cells in a 70 kg reference man, ratio about 1.3, bacteria about 0.2 kg).
Checked: 2026-10-17