Microorganisms are very small organisms or living things. Micro means small. Most can only be seen with microscopes. Microscopes are used to view tiny things. Microorganisms are everywhere on Earth. They’re in the water, in the air, on our skin, inside our bodies and in the soil. There are several different kinds of microorganisms.

Some microorganisms are beneficial to humans. Some are harmful. Germs or pathogens are microorganisms that can make us sick.


Bacteria are single-celled microorganisms that come in different shapes including rods, spheres and spirals. Some bacteria have tails called flagella that they can use to move around. Some bacteria are good and some are bad. There are far more types of good bacteria. Bacteria in our stomachs help us digest food. Bacteria are also used to make some types of food like yogurt, cheese and pickles. Bacteria in soil called decomposers break down dead plants and animals. This prevents dead materials from piling up all over the Earth.


A virus is a microorganism that can only replicate or make more of itself inside the cells of another organism. They can’t survive for long on their own. They depend on the cells of other living organisms to survive. The flu, the common cold, measles, mumps, chicken pox and many other illnesses are caused by viruses. The word virus comes from a Latin word meaning poison. No one can decide whether viruses are living things or not. They aren’t even included in the Six Kingdoms of Life that scientists use to categorize all living things.


Fungiaren’t either plants or animals but they have some characteristics of each. Yeast and mold are types of fungi. Yeast is used to make bread rise. Some kinds of fungi can cause infections in people. Athlete’s Foot is a skin infection caused by fungi. Mushrooms look like plants but they’re actually fungi. Plants can make their own food using a process called photosynthesis. Mushrooms can’t make their own food, so they’re not plants. They absorb nutrients from the soil.


Algae live in water or in damp places. Some have only one cell. They are called unicellular. Others have many cells. They are multi-cellular. Seaweed is a type of algae. Algae may look like plants but they’re not. They belong to a group of living things called protists. Some algae are microscopic, so they can only be seen with a microscope. Other algae, like seaweed, are large and easy to see.


Protozoa live in water or soil. They are unicellular, which means they have just one cell. Amoebas and paramecia are two well known types of protozoan. Protozoa are divided into four groups based on their size and shape and how they feed and move around. The four groups are Amoebas, Ciliates, Flagellates and Sporozoans.


Many archaea are extremophiles. Extremophiles can live in very extreme environments that no other organism could survive in, such as very hot geysers and areas with no oxygen.