Science Fair Experiment for Junior High
This experiment will give you a good idea of the fact that we share our world with microbes. Some bacteria are germs, some bacteria are helpful, but most are benign to humans. (Germs are microbes that can cause disease or illness and they may be bacteria or fungi.)
Agar may be purchased at an educational supply store or you may substitute unflavored gelatin mixed with a bullion cube.
Petri dishes may be purchased at an educational supply store or you may substitute sterilized small glass jars with lids. If you use glass jars with lids be sure to make 2 small punctures in the lid to admit air.
Problem
Which of the following household items have the most microbes?
(Suggested items include: sole of shoe, door knob, comb, remote control, telephone, refrigerator door handle, seat of toilet, desktop.)
Hypothesis
If I test common household items for microbes (bacteria and mold) then the shoe sample will grow the most (measured by counting colonies) because shoes touch everything on the ground.
Variables
Independent Variable: Common household items.
Dependent Variable: The numbers and kinds of microbes that grow depend on which common item is tested.
The things that must remain constant are:
1. growth time
2. growth temperature
3. amount of darkness/light
4. nutrient medium
5. cotton swab
6. container
7. humidity
Materials
8 cotton swabs
9 petri dishes or small glass jars with lids (Sterilize the glass jars by boiling in water for 15 minutes.)
sauce pan
1-cup measuring cup
1 teaspoon
distilled water
nutrient agar
1 glass jar with lid
1 brown paper bag
closet or other suitable dark storage place
Note: If gelatin and a bullion cube are substituted for nutrient agar, mix one packet (1/2 ounce) of gelatin with one bullion cube (beef or chicken) and one cup of distilled water. Heat just until boiling, stirring constantly until the powder dissolves.
Procedure
*Put 1 cup of distilled water and ½ teaspoon of nutrient agar in the sauce pan.
*Stir until the agar dissolves.
*Heat the mixture over medium heat until the liquid is just hot enough to boil.
*When foam appears on the surface of the liquid (right after it begins to boil) remove the pan from the heat.
*Pour the nutrient liquid in 9 equal parts into the 9 petri dishes. One petri dish is the control’ (the dish you do not streak).
*Close the containers with the liquid nutrient agar mixture in them and wait about 30 minutes for the agar to gel.
*Prepare each cotton swab by dabbing the cotton in distilled water to dampen the tip. Rub the swab back and forth on the surface of the desired item (shoe, door knob, remote, etc.).
*Raise the lid of the agar container just long enough to streak the swab across the agar’s surface. Leaving the lid open longer will allow airborne microbes to land on the surface of the agar and contaminate the experiment.
*Hold the swab at an acute angle to the surface of the gelled agar and lightly rub the swab across the surface of the gel in a zigzag pattern.
*After collecting all 8 samples, close the containers and store upside-down in a paper bag to ensure darkness and wait about 3 days for visible mold and/or bacterial growth. (The lids of the nutrient containers will have condensation. If the containers aren’t stored upside-down the condensate will ‘rain’ onto the surface of the agar which may drown the microbes and prevent growth.)
Data Table
Item / Number of Colonies
Shoe
Door knob
Comb
Remote control
(your choice)
(your choice)
(your choice
(your choice)
Control
Results
Interpret the data you collected here. Compare what happened in terms of growth of colonies, item tested, size, shape and color of colonies, odor, etc.
Conclusion
Write this section like a report. Restate your hypothesis and describe what you think about the experiment either proving or refuting your hypothesis. Use examples from your experiment to explain why your hypothesis was proven or disproven. You should add your thoughts on how you could improve this experiment if you performed it again.
