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Microorganism

From Tilcayo Cats, the lean encyclopaedia by Tilcayo Knowledge Systems
Tilcayo Cats original · British Bees · Sources and editorial assessment · Originals retention rule

A microorganism is a living thing that is too small to see with the unaided eye. Most microorganisms consist of a single cell. They include bacteria, archaea, fungi, protozoa and algae.[1] Microorganisms and viruses together are often called microbes, and their study is called microbiology. Viruses are not usually counted as microorganisms, because they are not living cells.[1][1]

Discovery

Microorganisms were first described about 350 years ago by van Leeuwenhoek, using microscopes that magnified about 300 times.[2][2] He even described microorganisms living on a person's teeth.[3] Modern electron microscopes can magnify about 500,000 times.[2]

For a long time, many people believed that microbes arose from non-living matter, an idea called spontaneous generation. In the mid-1800s Louis Pasteur's experiments with nutrient broths gave the strongest evidence against it.[4] Before the idea that germs cause disease was accepted, people blamed illness on angry gods or bad air.[4]

Kinds of microorganism

Cells were long divided into two types. Prokaryotes usually have a single circular chromosome. Eukaryotes have several DNA molecules inside a nucleus surrounded by a membrane.[5] Work by Carl Woese on ribosomal RNA led scientists to divide all life into three domains: Bacteria, Archaea and Eukarya.[6]

Bacteria and archaea

Bacteria and archaea are both prokaryotes, without a true nucleus. Most are about 0.5 to 5 micrometres long.[7][8] Archaea look like bacteria, but they differ from them in important ways. Many of the enzymes they use to copy and read their DNA are more like those of eukaryotes, and their cell membranes are chemically different.[9] Some archaea live in extreme places, such as the very hot water around deep-sea vents called black smokers. Others live in soil, fresh water, seawater and the human body.[7] Microorganisms can attach to surfaces and form layers called biofilms, held together by a matrix of polysaccharides (complex sugars).[10]

Fungi and protozoa

The cell walls of fungi are made mainly of chitin, a substance made from glucose.[5] Paramecia and amoebas have no cell wall. They use contractile vacuoles to pump out extra water so that they do not burst.[11] The slime mould Dictyostelium lives as single amoebae that feed on bacteria, but when food runs short they gather together to form a many-celled "slug".[12]

Microorganisms in nature

Microorganisms have been on Earth for a very long time. Fossil stromatolites made by photosynthetic bacteria are about 3.5 billion years old.[6] According to the endosymbiotic theory, the mitochondria in eukaryotic cells came from bacteria that were swallowed by other cells, and chloroplasts came from photosynthetic bacteria.[13]

Only certain bacteria and archaea can take nitrogen from the air and turn it into forms that living things can use. Some do this on their own, and others live alongside plants.[14] Cyanobacteria take in carbon dioxide through photosynthesis.[14]

Helpful and harmful microorganisms

Microbes can be both helpful and harmful. Microbes in the gut help with digestion, nitrogen-fixing microbes help plants, and fermentation by microbes is used to make foods such as yogurt.[1] On the other hand, some fungi, bacteria, viruses and protozoa cause disease.[1] The Black Death of 1340–1400 was caused by a bacterium spread by fleas. It killed a third or more of the people of Europe, Asia and North Africa.[15] Malaria is caused by a protozoan that enters red blood cells,[16] and cholera is a disease caused by a bacterium.[17]

Viruses that infect bacteria are called bacteriophages, and most of them have a head and a tail.[18][19]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 3. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  2. ↑ 2.0 2.1 2.2 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 2. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  3. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 24. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  4. ↑ 4.0 4.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 25. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  5. ↑ 5.0 5.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 6. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  6. ↑ 6.0 6.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 8. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  7. ↑ 7.0 7.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 110. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  8. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 113. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  9. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 10. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  10. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 897. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  11. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 211. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  12. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 97. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  13. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 98. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  14. ↑ 14.0 14.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 23. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  15. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 27. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  16. ↑ Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. pp. 335–336. ISBN 978-1-119-45417-5. {{cite book}}: |access-date= requires |url= (help)
  17. ↑ Karp, Gerald; Iwasa, Janet; Marshall, Wallace (2018). Karp's Cell Biology (Global ed.). Singapore: John Wiley & Sons. p. 339. ISBN 978-1-119-45417-5. {{cite book}}: |access-date= requires |url= (help)
  18. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 2. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  19. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 238. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
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