BioEd Online

Teacher Background

Microbes grow and reproduce in habitats where no other organisms can survive. They can be found in hot springs and deep underground veins of water, in volcanic rock beneath the ocean floor, in extremely salty water in the Great Salt Lake and the Dead Sea, and below the ice of Antarctica. Not even radiation or high levels of deadly chemicals, such as lead or sulphur, can kill the hardiest of microbes, referred to by scientists as “extremophiles.” Most extremophiles are single-celled organisms similar to bacteria, called Archaea. Many classifications place Archaea (or archaebacteria) in their own kingdom or domain, instead of with bacteria.

Microbes also are found in more mundane places, such as on our hands, in the air and in soil. This activity is designed to help students understand the diversity of microorganisms present in our immediate surroundings and on our bodies. It also will teach students how to limit the spread of disease-causing microbes. In addition, students will observe examples of bacteria and fungi.

Bacteria are the most numerous living things on Earth. Each bacterium consists of a tiny cell that must be magnified at least 400 times to be visible. Even though individual cells are not visible without the aid of a microscope, bacterial colonies (clumps of bacteria) grow large enough to be seen clearly.

Yeasts are fungi. They are small, single-celled organisms that can reproduce asexually by producing buds. They are known for their ability to obtain energy from food sources through a process known as fermentation. Fermentation yields alcohol and carbon dioxide gas as byproducts. It is used in the production of alcoholic beverages, such as beer and wine, and in making bread and other baked goods.

Molds, which also are fungi, consist of long, tangled filaments. Hair-like masses of molds often contaminate bread and cheese. They also are important, but usually unnoticed components of soil.

In some instances, infections by bacteria or fungi can cause disease. Contamination by these organisms also can make food unsafe to eat. The slime found on food that has been in the refrigerator too long is made of clumps of bacteria and sometimes fungi. 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 restroom, changing diapers or playing with a pet. Some foods, especially meats and poultry, can have bacteria on their surfaces that can be transferred to other foods if utensils and cutting boards are not washed with soap and hot water after each use.

In the laboratory, bacteria are grown on substances called culture media. The medium usually contains an energy source, such as a sugar dissolved in water, plus other nutrients, such as nitrogen. Culture media can be in liquid form (usually called a broth) or gelatin-like (called a gel).

In this activity, students will grow microbes on a semisolid gel refined from algae, a medium often referred to as nutrient agar.

Objectives and Standards

Inquiry

• 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.

Life Science

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

• Some diseases are the result of damage by infection by other organisms.

• Populations of organisms can be categorized by the function they serve in an ecosystem.

Materials and Setup

Teacher Materials (see Setup below)

• 750 mL of nutrient agar (purchase as powder or bottled agar gel)

• 33 100-mm sterile, disposable Petri dishes (to prepare 30 dishes with agar and 3 dishes for templates for student drawings) 

• 36 small, resealable plastic bags

• Chlorine bleach solution (download the activity PDF for dailed safety informaiton)

• Cotton swabs, 100-count box

• Disinfectant (liquid soap or spray)

• Hot pads or pot holders

• Paper towels

• Resealable plastic bag, medium-size

Materials per Group of Students

• 5 prepared Petri dishes (one dish is the control)

• 4 sterile cotton swabs in a resealable plastic bag 

• Clean, empty Petri lid or dish (for use as a drawing template)

• Container of distilled or boiled water

• Magnifiers or low power microscopes

• Masking tape

• Permanent marker or wax pencil

• Colored pencils or markers

• 12 sheets of white paper for observations (3 per student)

• Graph or plain paper

• Group concept map (ongoing)

Material Options

• For convenience, commercially prepared Petri dishes may be purchased with agar already added.

• Slices of cooked potatoes can be used as an alternative to nutrient agar. Boil whole potatoes until almost soft. Using a clean, dry knife, cut potatoes into 1/4-in. slices. Place each slice in a Petri dish or a clean, resealable, plastic bag. To safely discard potatoes in the bags after the activity, pour about 20 mL of a 10% bleach solution into each bag. Seal and discard the bags.

Setup

1. Place four sterile, cotton-tipped swabs in a resealable plastic bag for each group.

2. If not using pre-poured Petri dishes, prepare the dishes in advance (one hour to one day before conducting the activity). Mix powdered nutrient agar following package directions. If using bottled agar gel, completely loosen the cap on the bottle of agar, set the bottle in a pan of boiling water or in a microwave oven, and warm it until the agar melts (about 60°C). If using a microwave oven, heat the bottle on high for 30 seconds. Use a hot pad to remove the bottle and swirl to mix the agar. Heat and swirl at 10 second intervals until the agar is completely melted. To avoid condensation in the Petri dishes, let the agar cool slightly before pouring it into the dishes.

3. Open each Petri dish slightly, pour in enough agar to cover the bottom, (approximately 1/8 in.), and immediately replace the cover. Let the agar cool and solidify, and then store the dishes upside down to prevent condensation.

4. During the activity, store sealed Petri dishes upside down in a dark, warm place (at or about 37ºC or 98.6ºF).

5. Have students work in groups of four.

Procedure and Extensions

Session 1: Getting Started

Time: 45 minutes

1. Ask students to share what they already have learned about where microbes might live and grow. Follow by asking, Do you think there are any microbes in this room? Where might they be? List students’ ideas on the board or overhead.

2. Follow by asking, How could we find out if any microbes are present in these places? Encourage students to share their ideas, reminding them of the activity in which they observed bacteria growing in yogurt. If not mentioned by students, suggest that the class could collect samples from different places, provide opportunities for microbes from the samples to develop, and observe the results.

3. Have each group of students select four places (or more, depending on the number of Petri dishes available) that they would like to test for the presence of microbes. Possibilities include the floor, a doorknob, unwashed hands, etc.

4. Have each group create a table with two columns: “Location Sampled” and “Predicted Results.” Students should record information on this chart as they collect samples. For example, a group might predict that a sample from the doorknob will have more microbes than a sample from the surface of the door.

5. Review "Safety Issues" with students. Then, give each group five Petri dishes. One dish will be a control. The remaining four dishes will be used to grow cultures (one per student). Have students label the bottom of all five dishes using masking tape and a marker, or by writing directly on the dishes using a permanent marker or wax pencil. (You may grow more than one culture per dish. Simply divide each dish in half or quarters, drawing lines on the outside with a permanent marker.)

6. Have students use a different clean cotton swab dipped in boiled or distilled water for each sample. You may want to have students think about why the water needs to be boiled or distilled. (Otherwise, the water may contain microbes.) Have students rub the moist swab several times over the area to be tested.

7. Instruct students to open the Petri dishes only enough to swab the gel surface. Tell them to rub the swab gently in a zigzag pattern over the surface of the nutrient agar without breaking the surface of the agar gel. Students may repeat the pattern in another direction. Have students close and seal the dishes by taping around the edges. Tell students that they will not be able to see streaks on the plate after swabbing. Have students rub (inoculate) the control dish with a clean, moist swab.

8. Collect used swabs from students and discard as instructed in "Safety Issues." Clean all work areas with paper towels and disinfectant.

9. Collect and store sealed Petri dishes.

Sessions 2–4: Follow-up

Time: 15 minutes for 3 days to observe cultures

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 the pre-assessment activity, “What Do You Know About Microbes?” 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.

Extensions

• Have students design additional experiments to test for the presence of microbes. They might examine water from different sources, compare washed vs. unwashed hands, or see which kinds of food grow the most kinds of microbes or spoil most quickly.

• Have students investigate what happens when similar samples are grown at room temperature and in the refrigerator. Based on their results, conduct a discussion about the importance of refrigerating leftover food.

Handouts and Downloads