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Microorganism Definition

A microorganism is a living thing that is too small to be seen with the naked eye. Examples of microorganisms include bacteria, archaea, algae, protozoa, and microscopic animals such as the dust mite.

These microorganisms have been often under-appreciated and under-studied. Indeed, until Anton von Leeuwenhoek invented the microscope, we did not know they existed! Until that time, it was thought that phenomena such as illness and food spoilage were caused by “vapors” or “spontaneous generation.”

Antonie van Leeuwenhoek
Antonie van Leeuwenhoek

Leeuwenhoek’s invention of the microscope soon led Louis Pasteur to realize that many diseases were caused by microorganisms – and to the practice of pasteurization, which kills microorganisms and makes our food products safe to eat today.

Now, we know that microorganisms are responsible for many things that happen in the world around us.

Microorganisms are found virtually everywhere, except for environments that have been made artificially sterile by humans. Even these must be constantly sterilized and carefully protected, lest microorganisms be tracked in from the outside world.

Microorganisms live in water, in soil, and on the skin and in the digestive tracts of animals. This is why all living things must have immune systems – while many microorganisms can be helpful to them, some can be harmful and cause disease.

Like all organisms, microorganisms play important roles in the ecosystems they inhabit. Here are a few of their roles.

Types of Microorganisms


Bacteria – now sometimes called “eubacteria” or “true bacteria” to differentiate them from archaebacteria – are the type of microorganism you probably hear about the most.

This is because they’re the type most likely to make you sick. Bacteria are the cause of most skin infections, and can also cause food poisoning, pneumonia, strep throat, and many other illnesses.

However, bacteria are also very helpful to humans. “Good bacteria” in our digestive tracts help us to extract nutrients from our food, and help to fight pathogens that could hurt us.


Archaea, or archaebacteria, were once thought to be part of the bacteria family. However, recent research has shown that they are much different from eubacteria, and may even be more closely related to us than they are to modern bacteria.

Archaea can be found in many of the same places as bacteria – in water, in soil, and inside our digestive tracts, where they help us to stay healthy.

However, archaebacteria can also be found in some unusual places – many are able to live in environments that are very hot, very cold, very acidic, or very salty.

This makes them a common finding inside hot springs and other places where other organisms cannot easily survive.


Several types of animals come in microscopic varieties, including:

  • Arthropods (dust mites, spider mites, microscopic crustaceans)
  • Rotifers (a type of zooplankton)
  • Loricifera (microscopic animals that live in ocean sediments)
  • Nematodes


Protozoa are a diverse group of unicellular eukaryotic organisms. Like bacteria and archaea, they are single-celled; but their cells resemble those of animals and plants more than those of bacteria or archaea.

Several dangerous human diseases including malaria, toxoplasmosis, giardia, African “sleeping sickness,” and Chagas disease are caused by protozoa.


Though some microscopic fungi can infect humans just like bacteria or protozoa, there’s one microscopic fungus that most humans like a lot: yeast!

Yeast is the fungus that is responsible for making baked goods rise; and for producing alcoholic beverages such as beer, wine, and liquor.

Yeast feeds on sugars found in foods and converts it into carbon dioxide – and, yes, ethyl alcohol. The carbon dioxide can make our breads and cakes fluffy; and the alcohol can build up to intoxicating levels, if yeasts are bottled with a high concentration of sugar.


Molds are microorganisms that share some properties of fungi, but are not true fungi.

These include pathogenic molds that infect plants and have caused devastating crop failures such as the Great Irish Famine of the 1840s.

They also include the fantastically weird class of slime molds – single-celled organisms that are capable of cooperation so impressive that, during one stage of their life cycle, many slime mold cells gather together and operate like a single organism.

Slime mold intercellular cooperation is so impressive that scientists have been using slime molds to study intelligence and problem-solving!


Microscopic algae were once thought to be plants, but recent studies have shown that algae don’t fit into the plant family. Instead, these single-celled photosynthetic organisms are thought to be relatives of the lineage that led to land plants.

Throughout history, algae have been important photosynthesizers. They likely evolved before land plants did, and helped to pump oxygen into Earth’s atmosphere along with their ancestors, the cyanobacteria.

Today algae can both help and hurt humans – some species clean water and produce oxygen, while others produce dangerous toxins that can end up in our seafood and drinking water.


There are many other microscopic organisms that scientists are struggling to neatly classify. Once, many microorganisms were lumped into one category called “protists,” but many scientists now believe this system was only useful for explaining that the organism didn’t fit into any other kingdom.

The kingdom “Protista” served as a sort of “miscellaneous” bin for eukaryotic organisms that scientists could not readily identify as plants, animals, fungi. The logic was understandable: when light microscopes are the only tool you have, most microorganisms look fairly similar to each other.

Upon genetic analysis, however, many members of the kingdom “protista” turned out to be more closely related to these other groups than to each other!

Examples of Microorganisms

Streptococcus bacteria

Streptococcus is a group of bacteria that causes illness in humans. As the name suggests, streptococcus bacteria is the cause of strep throat – and can also cause scarlet fever and, rarely, skin and muscle infections.

Streptococcus is a good example of the “dangerous” type of microorganism. On this list, we’ll discuss microorganisms that are both dangerous and helpful to humans.

Malaria Parasite (Plasmodium)

Malaria is the deadliest disease to humans on Earth today. Transmitted from host to host by mosquito bites, it causes serious symptoms such as fever, hemolytic anemia, and convulsions as the Plasmodium parasite reproduces inside the host’s body.

It may surprise you to learn that the Plasmodium parasite is not a bacteria – rather, it is a eukaryotic microorganism that reproduces sexually and undergoes a multi-stage life cycle.

Plasmodium demonstrates the diversity of microorganisms – which may be bacterial, eukaryotic, or even multicellular.

Lactobacillus rhamnosus

If Streptococcus is a “bad” bacterium, Lactobacillus is a “good” type of bacterium. Lactobacillus are bacteria that live in the guts of healthy people, and may help us fight off diseases such as the stomach flu.

Lactobacillus is found in many yogurts. Some people even take highly concentrated doses of Lactobacillus in the form of “probiotic” pills or capsules in hopes of staying healthy!


Cyanobacteria was one of the first types of life to evolve on Earth. Its modern descendant continues to play important ecological roles today. Cyanobacteria can turn carbon dioxide into oxygen – and it can turn unusable inorganic nitrogen into organic forms that can be used to make proteins and more!

Because cyanobacteria was one of the first organisms on the planet, it likely had to do those things for itself – there were no other nitrogen-fixing organisms around to partner up with.

Cyanobacteria is another great example of a “good” microorganism to which humans owe a great deal!


1. Which of the following is NOT true of microorganisms?
A. All are prokaryotic
B. All are eukaryotic
C. All are microscopic
D. All are harmful pathogens

Answer to Question #1
C is correct. The only thing all microorganisms have in common is right in the name – they are microscopic! Microorganisms include microscopic prokaryotes, eukaryotes, and organisms that can be both helpful and harmful to humans.

2. Which of the following is one reason why our understanding of microorganisms has changed a lot in the last 10 years?
A. For most of the 19th and 20th centuries, microorganisms could only be studied using light microscopes.
B. The advent of genome analysis allowed scientists to read organisms’ “source codes” and see which are related to each other.
C. Reading the DNA of microbes have shown that many assumptions made based on light microscope studies were not correct.
D. All of the above.

Answer to Question #2
D is correct. All of the above are reasons why our understanding of microorganisms has changed greatly in the last 10 years, and is still changing very fast!


  • Madigan M; Martinko J, eds. (2006). Brock Biology of Microorganisms (13th ed.). Pearson Education. p. 1096.
  • (n.d.). Retrieved April 23, 2017, from http://www.ucmp.berkeley.edu/archaea/archaea.html
  • “Algae Research”. National Museum of Natural History, Department of Botany. 2008. Archived from the original on 1 December 2008. Retrieved 19 December 2008.

Cite This Article

Biologydictionary.net Editors. "Microorganism." Biology Dictionary, Biologydictionary.net, 27 Apr. 2017, https://biologydictionary.net/microorganism/.
Biologydictionary.net Editors. (2017, April 27). Microorganism. Retrieved from https://biologydictionary.net/microorganism/
Biologydictionary.net Editors. "Microorganism." Biology Dictionary. Biologydictionary.net, April 27, 2017. https://biologydictionary.net/microorganism/.

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