BioEd Online

Teacher Background

Microbes are organisms too small to be seen with the naked eye. There are enormous variations in the kinds and sizes of microbes. This activity allows students to observe representatives of three different groups of microbes—bacteria, fungi, and protists—first hand.

First, students will observe bacterial cells in yogurt, which will be visible only as tiny rods (rod-shaped bacteria are called bacilli). Yogurt is created when milk is fermented by Lactobacillus and other kinds of bacteria. It has a slightly sour taste, is acidic, and stays fresh longer than milk. A yogurt recipe is included as an extension to this activity.

Students also will observe yeast, which are single-celled fungi. This group also includes organisms like mushrooms and molds. Fungi are not able to trap energy through photosynthesis and must feed on other organisms. Many fungi are important decomposers within ecosystems. Yeasts have numerous applications in food production, such as leavening in bread and fermentation for alcoholic beverages. Some kinds of yeast also cause diseases, such as diaper rash or thrush (a painful infection of the mouth and throat).

Finally, students will observe a paramecium. “Slipper-shaped” paramecia are among the largest microorganisms in the protozoan group, which are considered to be protists. Most of the 40,000 species of protozoa are found in aquatic environments or in moist soil. A few are parasites. Protozoa do not have rigid cell walls (such as those in the onion skin cells). Paramecia take in particles of food through an “oral groove” located on one side of the organism, and they use tiny hairs, called cilia, to propel themselves through water.

Objectives and Standards

Inquiry

• 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.

• All organisms are composed of cells—the fundamental unit of life. Most organisms are single cells; other organisms, including humans, are multicellular.

• Cells carry on many functions needed to sustain life.

Materials and Setup

Teacher Materials (See Setup)

• For paramecia, order a culture in advance, collect pond water, or make your own.

• For yeast mixture (prepare one day in advance):

- 250-mL container

- 150 mL of warm water

- package of dry baker’s yeast

- tsp of sugar

• 6–8 oz of plain, unflavored yogurt

• 6 sheets of cardstock (to make "Slide Preparation" cards)

• small dropper bottle of glycerin (one drop per slide will slow paramecia if they are overly active)

• permanent marker to label droppers

Materials per Group of Students

• microscope (one or more per group)

• samples of microorganisms in three small containers:

- 50 mL of pond water

- 20 mL of yeast mixture

- 20 mL of enriched yogurt

• 20 mL of tap water in a cup

• 3 plastic cover slips

• 3 plastic slides

• 3 droppers (one each for yeast mixture, pond water or paramecia, and tap water)

• toothpick (for yogurt)

• plastic tray (to hold materials)

• set of colored pencils or markers

• set of "Slide Preparation" cards

• science notebooks

• group concept map (ongoing)

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Setup

1. Order pond water, make your own (see “Making Pond Water”), or collect 500 mL of fresh pond water from a ditch or pond. (Look for standing water that has a greenish color; collect some of the algae and sediment.) OR create your own pond water culture, boil water, then let it cool. Add the cooled water to straw or dried grass. Cover it with a cloth or paper towel, and place the mixture in a warm, sunny spot for several days. When it turns cloudy or green, it’s ready.

2. On the day before class, mix 1 teaspoon of yeast and 1 teaspoon of sugar into a cup containing 250 mL of warm water.

3. To enrich the microbe count in yogurt, use an individual-sized container of plain, unflavored yogurt that is past the expiration date, or let it sit unrefrigerated overnight.

4. Make six copies of the "Slide Preparation" cards student sheet on cardstock. Cut out cards.

5. Place materials for each group on trays, which will be picked up by each group’s Materials Manager. Leave the containers with pond water and yeast mixture on the distribution table until needed. For each group, label one dropper or pipette for the yeast mixture, one for pond water or paramecia, and one for tap water. At least one microscope should be placed on each group’s table before class begins.

Optional: As an alternative to science notebooks, make and distribute clean copies of the "Magnification Observations" student sheet (see Lesson Media tab, above).

Procedure and Extensions

1. Ask students, Have you ever seen a microbe? What do you think different kinds of microbes might look like? Tell students that they will have opportunities to observe and compare different kinds of microbes: bacteria in yogurt, yeast cultures, and paramecia (or other pond organisms).

2. First, students will prepare and examine a slide to observe bacteria in yogurt. If necessary, demonstrate how to make a slide before having groups of students make their own. Students should follow the instructions on the Bacteria card to prepare and observe their slides. Have students draw what they observe. (Students will not be able to see any parts inside the bacterial cells, which will appear as tiny rods.) Be sure students record on their drawings the magnification at which they made their observations.

3. Ask, Were you able to see cells or groups of cells? Explain that students observed tiny rod-shaped bacteria, called Lactobacillus, that live on milk sugar (lactose).

4. Have students follow the procedures on the Yeast card to observe and draw yeast cells. Students will be able to observe many round yeast cells, some of which may be reproducing by budding. Ask, Were the yeast cells larger or smaller than the bacteria? (larger) What other differences did you notice?

5. Finally, have students observe the paramecium culture (or pond water, which may or may not contain paramecia).
   
   If you are using pond water that includes a variety of organisms, you may want students to examine the water with a hand lens before using a microscope. Students should follow the directions on the Paramecia card to prepare their slides.
   
   Have students draw one paramecium (or other pond organism). These organisms may be large enough for students to observe and label the cell nucleus and cell membrane. Students also may be able to see the cilia around the edge of each paramecium.
   
   Tip: A tiny drop of glycerin on slides with pond water will slow the movement of microorganisms so that they are easier to observe.

6. Conduct a class discussion or have each group create a table that summarizes the similarities and differences observed among the three kinds of microorganisms. Allow time for groups to add new information to their concept maps.

7. Have students research or discuss other types of bacteria, fungi, and protists.

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Extension

It’s easy to make yogurt using the following ingredients:

• 2 quarts of whole milk

• 1 cup of plain yogurt

• 1 cup of half-and-half

Bring milk to a boil in a very clean pot. (Greasy or dirty pots and utensils won’t produce the desired results.) Remove from heat and let stand until cool. Pour the cooled milk into a glass or pottery jar, bowl, or other glass container. Measure one cup of the milk and pour it into a medium-sized bowl. Mix in the yogurt and half-and-half. Slowly add the remaining milk, stirring gently. Place a lid on the container or cover it with plastic wrap.

Wrap the container in a blanket or heavy towel. Place it in a corner of the room where it will stay warm and undisturbed for about six hours. Then refrigerate for 10 hours.

If you prefer a more tart flavor, leave the container wrapped for eight or nine hours. For a sweeter, softer yogurt, leave the container out for about four hours. Always keep one cup of yogurt from the previous batch to use as a starter for the next batch.

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