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Author(s): Nancy Moreno, PhD, Barbara Tharp, MS, Deanne Erdmann, MS, Sonia Rahmati Clayton, PhD, and James Denk, MA.

The Variety and Roles of Microbes

Microbes live almost everywhere on Earth—including within and on other living organisms. They can be found in almost all climates, from extreme heat to freezing cold. Some microbes can make us sick, but only a very small percentage actually cause disease. In fact, many microbes—including most bacteria—are helpful. Much of the oxygen released into the atmosphere through photosynthesis comes from algae and blue-green bacteria. Many fungi and bacteria are essential for cycling nutrients in ecosystems and for acting as decomposers, breaking down dead organisms and the waste of living things.

We depend on microbes for food. What would a hamburger be without a bun, cheese and pickles (all of which are produced with direct assistance from microbes)? The cattle used for beef also rely on microbes to digest the tough grasses they eat. In our own intestines, microbes aid in digestion, make several essential vitamins and help prevent disease. 

This activity focuses on the diverse array of microbes and the functions they perform. Three kingdoms within the five king­doms system of classification are included here: Monera (bacteria), Protista and Fungi. The two kingdoms not considered to have single-celled, free-living individuals are plants (Plantae) and animals (Animalia). Many scientists now favor a three-domain classification system: Bacteria, Archaea and Eukarya. 

Viruses represent a special case within the groups of microbes. They do not have all of the structures necessary for independent life. They must invade and use living cells to reproduce. For this reason, many biologists do not consider viruses to be “living” organisms, and do not assign them to a kingdom or domain. 

Note: C. cassiicola causes “leaf spotting disease” in soybeans, cucumbers, tomatoes and tobacco. Other plants also are sometimes affected. 


1. Ask students, What are some different kinds of microorganisms? Do microorganisms have different kinds of roles? What are some examples? Discuss students’ ideas. Tell students they they will be looking at specific examples of materials and resources that involve microbes. 

2. Give each student group one bag of cards. Have students remove the set of 20 smaller cards, which describe roles performed by certain microbes. Instruct student groups to read, discuss and decide the best way to sort the cards into categories. Have groups make notes about how they made their decisions. Then, have a spokes­person from each group explain its rationale for sorting and discuss as a class. 

3. If students did not organize the cards by “role in food production,” “role in causing disease,” and “role in ecosystem/environment,” have them sort the cards into these new categories. 

4. Tell students that the cards also may be sorted by “kind of microbe” involved in each process. Instruct groups to take the four large cards (“Viruses,” “Fungi,” “Protists,” “Bacteria”) from the bag and read each card. Discuss the information and ask questions, such as, Which microbe group does not have members with cells? (viruses) Which groups have multi-celled members? (protists, fungi) Mention that all roles described on the small Microbe Examples cards are carried out by one or more members of the four groups described on the large cards.

5. Have students place their large Microbe Group cards on the table. Starting with the cards related to food production, have students use the clues on each small card to assign it to one or more Microbe Group cards. Students may notice that some roles are fulfilled by microbes belonging to two groups (e.g., cacao seeds are fermented by bacteria and fungi).

6. Discuss as a class. Point out that microbes related to food production are found in either the bacteria or fungi group. Ask, Are you surprised by this? What can we now say about microbes? Explain that most microbes are not harmful, and many are helpful. But some microbes, called pathogens, cause diseases in humans, animals, plants and other organisms. Ask, What do you know about disease? Instruct groups to select the small cards related to disease and place each one by the appropriate Microbe Group card. 

7. Ask students, What are the differences and similarities between the microbes involved in food production and the microbes that cause disease? What general statement could we make about microbes? (Microbes have many roles. Some are helpful; some are harmful.) Is it possible for the same microbe to be both helpful and harmful to humans or to another organism? (yes)

8. Repeat the sorting activity with the last group of small Microbe Examples cards: ecosystems. Discuss students’ groupings. Mention that while microbes often are invisible members of ecosystems, they play important roles in decomposition and in soils, and are important members of food webs. 

9. Ask, Why should we care about microbes? Discuss as a group. Have students add any new ideas to their concept maps. 

10. Have students sort and place the cards back in the plastic bags. 

Funded by the following grant(s)

Science Education Partnership Award, NIH

Grant Number: 5R25RR018605