Microorganism: Definition, Types & Classification

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  • 0:04 Micororganism:…
  • 1:35 Bacteria and Archaea
  • 2:46 Fungi, Protists, and Animals
  • 4:14 Are Viruses Microorganisms?
  • 4:51 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Microorganisms are tiny living things. There are so many microorganisms in the world. How do scientists classify and keep track of them all? Find out in this lesson!

Microorganisms: Definition & Types

Microorganisms are, as the name implies, microscopic organisms. They are living things that are too tiny to see with the naked eye. A microorganism can perform all the characteristics that any living thing can perform. It can move, get nutrients from the environment, maintain homeostasis, and evolve. This obviously happens on a smaller scale than it does for, say, an elephant, but microorganisms are definitely alive.

It can be hard to think about life that we can't even see. When you think about what living things are, you normally think of plants and animals. But there are many more organisms that are smaller than we can see. What are some examples of microorganisms?

Some microorganisms are prokaryotic, meaning they are made of small, simple cells, including bacteria and archaea. Bacteria might be the most obvious group of microorganisms, as you hear about them causing food poisoning on the news. Archaea are a group similar to the bacteria, but not as well known.

Some microorganisms are eukaryotic, meaning they are made from larger, more complex cells, with fungi and protists being the most common eukaryotic microorganisms. However, some animals can also be microscopic.

Microorganisms are classified based on what type of cell they have. They follow the same classification system that other organisms use. The domain is the largest group, and describes the basic cell type. Domains are broken into smaller kingdoms, which are then broken into smaller groups called phyla, and so on.

Bacteria and Archaea

Bacteria comprise an entire domain. One basic way to classify bacteria is whether they hold a special dye or not. The bacteria that hold the dye are called Gram-positive, and those that don't are called Gram-negative. Knowing this information can tell you what type of cell wall the bacteria has. This is important for doctors prescribing antibiotics.

Bacteria can also be classified based on their shape. Bacteria can be spherical, rod-shaped, or spiral. Spherical bacteria can also be classified based on formations. They can remain as individuals, join together in pairs or chains, or even form large clusters.

Archaea are another type of prokaryotic microorganism, but they have a domain separate from bacteria. Archaea have a slightly different cell wall, which helps differentiate them from bacteria. In addition, archaea are mostly found in extreme environments. They live in environments that are too hot, too cold, too salty, too acidic, or too alkaline for other living things. Humans don't interact with these microorganisms very much, because we can't inhabit the same places. Archaeans are classified based on where they live and what they use as a food source.

Fungi, Protists, and Animals

Some microorganisms have larger, more complicated cells. These are the eukaryotes. Fungi, protists, and animals can all be microscopic. Microscopic fungi include mold, yeast, and mildew. Some of these, such as mold and mildew, can grow big enough to be seen with the naked eye. Fungi are classified based on how they reproduce. In general, fungi release spores, which are like small, single-celled baby fungi. How these spores are produced helps determine how a fungus is classified.

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