BioEd Online

Teacher Background

Bacteria are the most numerous of all things living on our planet. However, they are so tiny that it is not possible to see one without the aid of a microscope. Most bacteria must be magnified at least 400 times before they can be observed. Each bacterium (a single bacteria) consists of one cell capable of reproducing very rapidly. In fact, one bacterium cell can produce millions of others in just one day.

Bacteria are essential in many ways. They are important decomposers in almost all ecosystems. Photosynthetic bacteria (also known as blue-green algae) are vital producers in aquatic ecosystems. Bacteria in the intestines of animals help break down some large food molecules during digestion.

Bacteria also can cause serious problems with food. Since bacteria are everywhere, it is easy for food to become contaminated by bacteria and begin to spoil. The slime you see on food that has been in the refrigerator too long is made of clumps of bacteria and, sometimes, fungi as well. Eating spoiled food can make humans and other animals sick.

Bacteria can be transferred to food when people do not wash their hands after using the bathroom, changing a diaper, or playing with pets. Some foods, especially meats, can have bacteria on their outside surfaces. These bacteria can be transferred to other foods if knives, spoons, and cutting boards used in preparing them are not washed with soap and water.

This activity allows students to observe bacteria and to compare relative amounts of bacteria living in different parts of the home, classroom, or school.

Objectives and Standards

Concepts

• Bacteria are everywhere.

• Bacteria need food to grow.

• Bacteria are important decomposers, but they also can cause many different kinds of diseases.

• Bacteria are a major source of food contamination.

Science, Health, and Math Skills

• Designing an experiment

• Making observations

• Drawing conclusions

Materials and Setup

Materials per Student Group

• 6 cotton swabs

• 3 sterilized petri dishes (see Setup)

• distilled water or boiled water (for swabs)

• masking tape for labeling and sealing petri dishes

• nutrient agar (order from a science supply vendor)

• pens or markers

Setup

1. Prepare the petri dishes for students. Warm the nutrient agar until it melts (about 60° C) in a pan of boiling water or in the microwave. To avoid condensation in the dishes, let the agar cool slightly before use. Open each petri dish, pour in enough agar to cover the bottom, and immediately replace the cover. Let the agar solidify before use. Store the petri dishes upside down.

2. As an alternative to using agar, bacteria also can be grown on potato slices. Boil whole potatoes until almost soft. Using a clean, dry knife, cut potatoes into 1/4-inch slices and place each slice in a petri dish or clean resealable plastic bag. Prepare the petri dishes for students. Warm the nutrient agar until it melts.

3. Have students work in teams of 4 to plan and carry out their experiments.

Safety

• Most bacteria are harmless to healthy people. However, because some kinds of bacteria can cause disease, it is important that the petri dishes remain closed after students have started the cultures.

• Students should not collect or test saliva, tears, or other body fluids.

• Dispose of used cotton swabs by placing them in a resealable plastic bag. Cover swabs with a 10% bleach solution (10 mL chlorine bleach mixed with 90 mL water). Seal the bag and discard.

• Dispose of cultures immediately after the activity. Carefully remove the tape used to seal each dish and place each closed petri dish in a separate, resealable plastic bag. Pour about 20 mL of a 10% bleach solution in the plastic bag. Seal the bag. Through the sides of the closed bag, loosen the cover of the petri dish enough to allow the bleach solution to move inside and completely cover the contents of the dish. Dispose of the plastic bag and its contents in the trash.

• Follow all district and school science-laboratory safety procedures. It is good laboratory practice to have students wash hands before and after any laboratory activity. Clean work areas with disinfectant.

Procedure and Extensions

Time

Two 30-minute sessions; three 10-minute observation sessions on the following consecutive days

Session 1: Setting up

1. Tell students that they will be learning about bacteria—tiny microorganisms present everywhere. Ask students to mention what they know or have heard about bacteria. List their ideas on the board.

2. Point out that bacteria are a major source of food contamination, and that students will be investigating where bacteria might be present. Ask, Can we see where bacteria are? How might we be able to find out where the most bacteria are in the room (school, etc.)?

3. Tell students that one way to study bacteria is to let them grow until they form a clump that is large enough to see. Mention that they will be finding and growing bacteria.

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, may also 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).

Extensions

• Design additional experiments to test for the presence of bacteria. You might test water from different sources or see which kinds of food grow the most kinds of bacteria or become spoiled most quickly by bacteria.

• Have students investigate what happens when similar samples are grown at room temperature and in the refrigerator.

Handouts and Downloads