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Author(s): Nancy Moreno, PhD, and Barbara Tharp, MS.

Growing and Observing Bacteria

Procedure (cont.)
Session 1: Setting up

4. Have students in each group select two places that they would like to test for the presence of bacteria. Possibilities include the floor, doorknob, unwashed hands, rinsed hands, hands washed with soap and water, etc.

5. Have the groups write descriptions of the places they plan to test and to write predictions about what they expect to find. For example, a group might predict that a sample from unwashed hands will have more bacteria than from washed hands.

6. Give each group three petri dishes. One dish will be a control. The remaining two will be used for sampling. Students should label all three dishes.

7. Direct students to sample the areas they have chosen using clean cotton swabs dipped in distilled or boiled water. They should rub the swab several times over the area to be tested and then gently rub the swab in a zig-zag pattern over the surface of the gel mixture in the bottom of the petri dish. Instruct students to open the dishes only enough to swab the gel surface. The control dish should be rubbed (inoculated) with a clean, moist swab.

8. Tape the dishes closed for students. Store the dishes upside down.

Session 2: Observations

1. If possible, have students observe the cultures every day for 1–3 days. After about three days, have students make detailed observations. Ask, What has changed inside the petri dishes? Bacteria will discolor the surface of the gel and form smooth, wrinkly or slimy blotches (called colonies) of different colors. Fungi, which form fuzzy colonies, also may be present.

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

3. Next, have students decide whether some sample sources had more bacteria than others by counting the number of colonies and/or by comparing the sizes of colonies. Have them record their observations and conclusions. Have the groups share their results with the rest of the class.

4. Based on the results, have students decide which locations have the most bacteria, and which the least. Ask, If there are bacteria all around us, why aren’t all of us sick? Do all bacteria make us sick? What about the gel in the petri dishes—would you want to eat it? Do you think that it is good to have bacteria growing in our food?

5. Help students understand that contamination of food by bacteria can cause serious health problems. Ask for suggestions on how to keep food clean. Possibilities include: using clean hands and utensils for food preparation, keeping food covered and refrigerated until used, and cooking food thoroughly to kill bacteria that might be present (see “Bacteria Busters!” PDF).

Note: For more information about food and food safety, including free food safety charts, visit the “Keep Food Safe” section of the website.

Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932

Houston Endowment Inc.

Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education