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The Variety and Roles of Microbes

The Variety and Roles of Microbes
  • Grades:
  • Length: 60 Minutes


Students use sets of cards to categorize microbes’ roles and uses and learn that some microbes can share characteristics with more than one group.

This activity is from The Science of Microbes Teacher's Guide and is most appropriate for use with students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

The guide is available in print format.

This work was developed in partnership with the Baylor-UT Houston Center for AIDS Research, an NIH-funded program.

Teacher Background

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 (algae and protozoans), and Fungi (mushrooms, yeast, and mold). 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.

Objectives and Standards


  • Identify questions that can be answered through scientific investigations.

  • Think critically and logically to make the relationships between evidence and explanations.

  • Recognize and analyze alternative explanations and predictions.

  • Communicate scientific procedures and explanations.

  • Use mathematics in all aspects of scientific inquiry.

  • Develop descriptions, explanations, predictions, and models using evidence.

  • Use appropriate tools and techniques to gather, analyze, and interpret data.

Life Science

  • Living systems at all levels of organization demonstrate the complementary nature of structure and function.

  • Disease is a breakdown in structures or functions of an organism. Some diseases are the result of intrinsic failures of the systems. Others are the result of damage by infection by other organisms.

  • Millions of species of animals, plants, and microorganisms are alive today. Though different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry.

Science in Personal and Social Perspectives

  • Students should understand the risks associated with natural hazards (fires, floods, tornadoes, hurricanes, earthquakes, and volcanic eruptions), chemical hazards (pollutants in air, water, soil, and food), biological hazards (pollen, viruses, bacteria, and parasites), social hazards (occupational safety and transportation), and personal hazards (smoking, dieting, and drinking).

Materials and Setup

Teacher Materials (see Setup)

  • 12 sheets of cardstock (to prepare cards)

  • 6 resealable plastic bags

Per Group of Students

  • set of 4 Microbe Groups cards and 20 Microbe Examples cards

  • group concept map (ongoing)


  1. Copy the "Microbe Groups" and "Microbe Examples" pages on cardstock. Cut out and make 6 sets of cards (4 large and 20 small cards per set). Place each card set in a resealable plastic bag.

  2. Have students work in groups of 4.

  3. Optional: Change the enlargement setting on a photocopier to 129% and copy pages onto 11-in. x 17-in. paper.

Procedure and Extensions

  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 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 the group's rationale for sorting and discuss to the 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 may also 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 Microbe Examples 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 are often 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.


  • Bring to class examples of the foods used in this activity. Or, have students bring different foods produced by using microbes and/or recipes that use microbe-produced foods.

  • Have students investigate other common foods produced with the aid of microbes, such as root beer (yeast), vinegar (bacteria), pickles (bacteria), or cheese (bacteria, and sometimes fungi).

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Science Education Partnership Award, NIH

Science Education Partnership Award, NIH

Grant Number: 5R25RR018605