Materials Included In Kit

Additional Materials Required

Safety Precautions

After use, Petrifilm™ Aerobic Count Plates will contain viable bacterial colonies. Do not separate or handle the plates unnecessarily. Treat them using sterile techniques. Exercise extreme caution when working with potentially harmful microbes.

Disposal

Soak the plates in 10% bleach solution for at least one hour or autoclave them before solid waste disposal. Too much caution cannot be exercised when handling culture materials and students should be trained in aseptic techniques. Always clean up work areas and wash hands after working with microbiological materials.

Teacher Tips

• 3M^® Petrifilm™ Aerobic Count Plates (Publication No. 10162) suggests using the direct contact method and outlines air sampling methods and swab contact methods, which may be useful in extending this activity.

• If an incubator is not available, a heating pad or other warming device may be used to incubate the plates. The microbes will grow at room temperature but will likely require 2–3 days depending upon the ambient temperature.
• Store unopened Petrifilm Aerobic Count Plates at or below 46 °F (8 °C). After opening, return unused plates to the foil pouch. Seal pouch by folding and taping the open end. Store resealed foil pouch in a cool, dry place. Use plates within one month after opening. Exposure of Petrifilm Aerobic Count Plates at temperatures and/or humidities above 75 °F (25 °C) and 50% relative humidity can affect the performance of the plates.

Answers to Questions

1. Draw the results on your plate in as much detail as possible. Describe the colonies in as much detail as possible.
   

   Answers will vary. Descriptions of colonies should include colors and shapes.
   

2. Compare your results with other locations. Where were microbes most prevalent? least prevalent?
   

   Answers will vary.
   

3. How might you treat your test surface to get rid of microbes? How could you prove you had gotten rid of them?
   

   A surface may be treated using an antimicrobial cleaner which is proven to kill bacteria. You could prove you killed them by testing the surface in the same fashion as it was originally tested in this laboratory.

References

Petrifilm materials are a 3M^® manufactured product and are to be used for non-commercial research and development purposes only.

Introduction

We are constantly being reminded about microbes in our surroundings. News broadcasts highlight spoiled food and disease outbreaks. Commercials appeal to our need to clean with antibacterial mouthwashes, soaps and other cleaners. How effective are these products? Where do bacteria normally live?

Concepts

• Bacterial colonies

• Sterile

Background

Microbes are everywhere. We are surrounded by them and bombarded by them, but we don’t see them. They are small in size but are present in enormous numbers. A pinch or two of garden soil contains billions of bacteria and the number in your mouth at any time is greater than the number of humans who have ever lived.

Microbes play very important roles in the living world. They are primarily responsible for the cycling of gases in the atmosphere. They are essential to the world food web due to their role as decomposers—breaking down complex organic compounds into inorganic materials that can be used by plants. Bacteria also serve as a food source for many organisms as well as other microorganisms.

Although some microbes cause disease, the vast majority are beneficial to humans. When working with microbes, biologists always assume that the microbes are pathogenic (disease-causing) and, therefore, handle them with great care. Precautions are taken because the microbes are too small to see with the naked eye and are, therefore, difficult to positively identify. Biologists have developed sterile techniques for handling microbes that keep the microbes confined in growth chambers and out of touch with humans. Biologists allow the single microbes to reproduce until they form individual colonies consisting of millions of bacterial cells. The size, shape, color and number of colonies can then be studied and compared from one area to another.

In this activity, Petrifilm will be used to grow microbes collected from various places in the environment. After incubation the growth patterns of the resulting colonies will be compared and inferences about the number and types of microbes present will be made. Identification of specific microbes will not be possible and, therefore, all will be treated as if they were dangerous. Follow the procedures carefully and good luck microbe hunting!

Materials

Safety Precautions

Once the microbes are collected on the Petrifilm plate, do not open the plate again. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Place the Petrifilm Aerobic Count Plate on a flat surface. Carefully peel open the Petrifilm plate being careful not to touch the nutrient gel with your fingers (see Figure 1).

2. With a pipet perpendicular to the Petrifilm plate, place 1 mL of bottled water onto the center of the bottom film (see Figure 2).

3. Release the top film; allow it to drop. Do not roll the top film down (see Figure 3).

4. With ridge side down, place the spreader on the top film over the water (see Figure 4).

5. Gently apply pressure on the center of the spreader to distribute the water over a circular area. Do not twist or slide the spreader, simply apply a gentle, downward pressure to the spreader (see Figure 5).

6. Lift the spreader (see Figure 6).

7. Label your Petrifilm plate with a permanent marker along the top edge of the film. Set the Petrifilm plate aside for at least four hours at room temperature to allow the gel to solidify.
8. Determine the place(s) your team will sample. Door knobs, sink handles, desk tops, cafeteria areas or any other surfaces in your environment are good areas to test. Coordinate with other teams so a variety of places are sampled.
9. When the gel is solid, carefully lift the top film portion of the hydrated plate. Avoid touching the circular growth area. The gel will adhere to the top film.
10. Allow the circular gel portion of the top film to contact the surface being tested. Rub fingers over the outer film side of the gelled area to ensure good contact with the surface (see Figure 7).

11. Lift the film from the test surface and rejoin the top and bottom sheets of the Petrifilm plate. Do not open the plate again.
12. Use a disinfectant solution to wash the area where the sample was taken. This precaution is taken since some gel was probably left behind and microbial growth should not be unnecessarily encouraged.
13. Incubate your Petrifilm plate as directed by your instructor for 24–48 hours.
14. Use the Microbe Hunting Worksheet to record your observations. Compare your plate with the control plate provided by your teacher. The control plate will identify any possible microbes that may be present in the bottled water. Record your sampling location. Draw sketches and write descriptions of the microbe colonies you have grown. Answer the question on the bottom of the worksheet.
15. Your instructor will sterilize the Petrifilm plates from this experiment before disposing of them. This will guarantee that no pathogenic microbes are spread. Carefully follow your teacher’s instructions for disposal.

Student Worksheet PDF

10190_Student1.pdf