Microorganisms
Some of the most important living things on Earth are far too small to see. Microorganisms are everywhere, in the soil, the air, and inside your own body, and they can cause disease, spoil food, or help keep whole ecosystems running. Knowing the main types of microorganisms is useful and commonly tested.
This lesson introduces microorganisms and the main groups.
Microorganisms are living things too small to see without a microscope. The main types are bacteria, viruses, fungi (like yeast and mold), and protists. Some cause disease, but many are helpful, breaking down waste, making foods, and recycling nutrients in ecosystems.
What are the main types of microorganisms?
Bacteria are single-celled organisms without a nucleus, found almost everywhere. Viruses are even smaller and are not made of cells; they can only reproduce inside a living host. Fungi include microscopic forms like yeast and mold, which absorb nutrients from their surroundings. Protists are mostly single-celled organisms with a nucleus, such as amoebas and many kinds of algae. Only a special microscope reveals any of them.
| Type | Key feature |
|---|---|
| Bacteria | Single cell, no nucleus |
| Viruses | Not cells; need a host to reproduce |
| Fungi | Absorb nutrients; yeast and mold |
| Protists | Single cell with a nucleus |
Are microorganisms harmful or helpful?
It is easy to think of microbes only as germs, but most are harmless or helpful. Bacteria in the soil and in your gut break down material and aid digestion. Yeast makes bread rise and ferments drinks. Microbes are the main decomposers in ecosystems, recycling dead matter into nutrients that plants can use. Only a minority are pathogens that cause disease, though those get most of the attention.
Watch: A Short Video Lesson
Magfar Online School walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for microorganism questions
- Recall the four main types: bacteria, viruses, fungi, protists.
- Remember viruses are not cells and need a host.
- Bacteria are single cells without a nucleus.
- Know that most microbes are helpful decomposers, not germs.
- Only some microorganisms are disease-causing pathogens.
Practice questions
- What tool is needed to see microorganisms?
- Which microorganism is not made of cells?
- Give an example of a helpful fungus and what it does.
- What role do microbes play as decomposers?
- Do bacteria have a nucleus?
- True or false: all microorganisms cause disease.
Answers:
- A microscope.
- A virus.
- Yeast, which makes bread rise and ferments drinks.
- They break down dead matter and recycle nutrients for plants.
- No.
- False. Most are harmless or helpful.
Where this fits
Microorganisms include the pathogens from immunity, pathogens, and disease and the decomposers in energy flow and the cycles of matter. They belong to the smallest organism groups. Find all topics on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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