Grade Level: 6th - 8th; Type: Biology
To determine which room in my house has the greatest number of bacteria
- What are microorganisms?
- What are bacteria?
- Where bacteria are commonly found?
- What is agar agar?
- Under what kinds of conditions do most bacteria grow?
- Are there useful bacteria? Cite examples.
- Are there harmful bacteria? Cite examples.
- How do we culture bacteria in the lab?
- Why do we refrigerate the Petri dishes when they arrive?
- Why do we tape the Petri dishes after exposure?
- What precautions should we take to preclude the spread of bacteria?
- What is a control and how is it used?
On the information level, this experiment serves to acquaint students with basic information on the growth of bacteria, on their prevalence in the air around us and in our total environment and on the conditions under which bacteria can readily grow and multiply. In addition, on the processing level, student learn to use the scientific method and to construct a hypothesis, to use a control, to identify variables, to gather and analyze data, formulate a conclusion and produce a report including a bibliography documenting their arm chair research.
- 10 Petri dishes containing sterilized, nutrient agar (can be purchased directly from Carolina Biological Inc.)
- Plastic gloves
- Disinfectant bleach
- Wax pencil
- Masking tape
If you do not have access to a school incubator you can make your own very easily, you will need:
- 20 gallon aquarium
- Heavyweight clear plastic tape
- Small lamp with a 75 watt bulb
- Thermometer (0-100C)
IMPORTANT NOTE: Refrigerate your Petri dishes containing the agar when you receive them. Store them upside down. Make sure the cover is at the bottom and the agar media is on top.
- Gather all the materials you will need for this project.These include the 10 Petri dishes, the plastic gloves, the sterile swabs, the bleach, the wax pencil, and the plastic bags to discard dirty swabs, the making tape and scissors.If you are not using the schools incubator, you will need the aquarium, the clear plastic, the tape, small lamp plus a 25 watt bulb and the Celsius thermometer (0-100). Include a camera if you wish to take photos of the Petri dishes containing the bacterial colonies.
- When ready to start the experiment, take the Petri dishes out of the refrigerator and let them come up to room temperature. This should take about an hour.
- Make 3 copies of the data chart provided below so that you may readily record your observations for each of the three days.
- Put on your apron, gloves and safety glasses.
- Using the masking tape and your wax pencil, make labels for each room of the house in which you are going to test for bacterial growth. Also, make an equal number of labels with the word control. Attach them to your Petri dishes. You now have an equal number of labels of the rooms you will check and a control for each one.
- Now you are ready to go to each room in your house or apartment and place two Petri dishes in the middle of the room on the floor. Open one Petri dish exposing it to the air in the room and keep the other taped shut. This other, the closed one is the control.Note, if you are testing 4 rooms, you have four open Petri dishes and 4 closed control Petri dishes.
- Allow for an exposure period of one day
- Borrow an incubator from your school or construct your own by using the following directions provided in step 9. It is easy! You will need it in step 10.
- To make your instant incubator, take the aquarium and turn it so that the opening faces the front, not the top. Use the heavy clear plastic to make a hanging door over the opening. Tape it on. Place the 75 watt bulb in the small lamp and place the lamp into the aquarium. Have the cord come out of the front. Place the thermometer inside the aquarium so that you can readily read the temperature and do not need to move it. Done! The temperature you need is 32 degrees or 90 degrees F.
- Now, put on your safely equipment, goggles, gloves and apron, close all of the open Petri dishes and seal them with masking tape. Do the same with the controls. You may wish to take photos of all of them. Look closely at each one and record your observations on your data chart.
- Put all of the Petri dishes into the incubator. Store them upside down. Keep them there for 3 days. Each day observe and record your findings in your data chart.Take photos if you wish or draw sketches of the visible colonies.
- When you are ready to discard the Petri dishes use bleach on them and place them in plastic bags .Have them soak in the bleach overnight and then discard.
- Write up your report. Make certain to include your responses to the research questions as well as any other data that you gathered.If you have taken photos, use them on your display board. Do not bring the Petri dishes plus colonies of bacteria to the fair! Use your sketches or photos!
- Congratulations on a job well done!
Open Petri Dishes Sketches
Terms/Concepts: Microorganisms; Bacteria; Colonies; Agar agar; Gelatin; Nutrients; Incubator
- Lovine, J . Constructing an Incubator, “The Amateur Scientist” in Scientific American. June, 1994 pp.108-111. (This matrial may be duplicated by teachers for classroom use.)
- Morholt, E, Brandwein, P., Joseph, A., A Sourcebook for the Biological Sciences, Harcout, Brace & World, 1966, p. 449 – 451.