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

Observing Results

Sessions 2-4: Follow-up

1. Distribute clean Petri lids or dishes. Have each student use a dish as a template to draw three separate circles, labeled “Day 1,” “Day 2” and “Day 3.” Have each group member observe and draw one of the group’s cultures each day. Ask one group member to prepare an additional sheet for observing the control. Students should take turns making control observations. 

2. Have students observe the cultures daily for 1 to 3 days. If possible, have them use a low power microscope to observe the cultures through the lids of the dishes. Do not allow students to open the Petri dishes. 

3. Conduct a class discussion. Ask, What has changed inside the Petri dishes? (Bacteria will discolor the surface of the culture medium and form smooth, wrinkly or slimy circular blotches, called “colonies,” of different colors. Molds, which form fuzzy or felt-like colonies, also may be present.)

4. Have students decide how many different kinds of organisms might be growing on their gels, based on differences they can observe. Do not allow students to open the dishes. 

Some common microorganisms that might be present include fuzzy green Penicillium mold, black fuzzy or hairy bread mold, or various circular white, dark or colored colonies of bacteria. Yeast colonies usually are white. It is not important for students to be able to name all the microbes.

5. On Day 3, have students count the number of colonies, or measure and compare diameters of the colonies on their observation sheets. Have students decide which sample sources had the most microbes. Students’ drawings from all three days also can be used to estimate microbial growth by reviewing changes in the number or size of the colonies over time. 

6. Have each group prepare a brief summary comparing its observations with its chart of sample locations and predicted results. Have groups share their summaries with the rest of the class. 

7. Based on these reports, have students answer the question posed at the beginning of the activity: Are there any microbes in the room? If so, where are they? Promote discussion by asking questions, such as, If there are microbes all around us, why aren’t we all sick? Relate students’ findings to Activity 1, in which fluorescent powder was used to simulate microbes on students’ hands. 

8. Also, discuss the multiple roles of microbes in the environment. Ask, Have you ever seen any colonies of microbes (particularly bacteria and molds) growing on food, on damp surfaces, or in natural environments? What do you think is happening when microbes grow on something? (The microbes are using the substance as a food source.) Discuss the important roles of microorganisms as decomposers of dead organic material in ecosystems. 

9. Allow students time to add to their concept maps. 

Funded by the following grant(s)

Science Education Partnership Award, NIH

Grant Number: 5R25RR018605