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A comparison on the contamination of wet and dry foodFeatured science projectScience project video


Materials required for this science fair project:

-    8 agar Petri dishes
-    80 disinfected swabs
-    1 bottle of disinfected water
-    1 box of cheese crackers
-    1 cucumber
-    1 knife
-    1 chopping board
-    1 box of sterilized rubber gloves
-    1 stopwatch
-    2 sterilized plastic bags


1.    The independent variable in this science fair project are the locations where the cheese cracker and cucumber slice were placed – kitchen sink, toilet floor, car porch or garden grass. The dependent variable is the amount of bacterial growth in the Petri dishes, which is determined by counting the number of bacteria colonies. The constants (control variables) are the temperature of the room, the amount of sunlight, and the ingredients used to make the agar in the Petri.

2.    Store the eight Petri dishes in a refrigerator. Before the start of the experiment, take the Petri dishes out of the refrigerator and allow them to warm to room temperature. Label the Petri dishes as wet sink, wet floor, wet porch, wet grass, dry sink, dry floor, dry porch and dry grass.

3.    Wear the sterilized rubber glove and wash the chopping board and knife using the sterilized water. Wash the cucumber and cut it into slices before keeping it in the sterilized plastic bag. Next, open the box of cheese crackers and transfer the crackers into another sterilized plastic bag.

4.    Using a new sterilized rubber glove, drop a slice of cucumber into the kitchen sink. Start the stopwatch and pick up the slice of cucumber after 20 seconds. Rinse a swab with disinfected water before rubbing it over the surface of the cucumber slice. Swab over the surface of the Petri dish labeled as wet sink. Repeat the procedure for ten times, each time using a new slice of cucumber and a new swab.

5.    Repeat procedure 4 using a piece of cheese cracker biscuit. Make ten dots using disinfected swabs in the Petri dish labeled as dry sink.

6.    Repeat procedures 4 and 5 by placing sliced cucumbers and cheese crackers on the toilet floor, car porch and garden grass.

7.    Keep the eight Petri dishes in a cool and dry place for the next three days. This is to allow the bacteria to grow. Count the number of bacteria colonies  after three days and record in the table given below.

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Complexity level:
Project cost ($):
Time required:
Preparation will require one day, while observation and recording of data will be carried out over a period of three days
Material availability:
Access to laboratory equipment (eg. petri dishes)
Safety concerns:

Always follow laboratory safety guidelines and always practice sterile technique when handling microbes. Never have any food or drink at your workstation and always thoroughly wash your hands with disinfectant soap or alcohol before leaving your workstation. Always dispose of used material in a biohazard bag. If none are available, the bacteria should be destroyed with bleach before being disposed of. Proper safety measures must be followed when handling the knife.