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The materials required for this science fair project are:

  • 7 PET Bottles
  • 7 liters of Tap Water
  • Escherichia Coli bacteria culture
  • 7 Agar plates in Petri Dishes
  • 7 Syringes
  • Sharp object, e.g. penknife, blade etc.


1.    For this experiment, the independent variable is the opacity of the bottle. The dependent variable is the quantity of E.Coli colony present in the water. The controls for this experiment are the source of water used, the amount of sunlight exposed to and the amount of E.Coli used in each bottle.

2.    Prepare 7 clean PET bottles for the experiment.

3.    Using the sharp object, scratch the outer surface of 4 bottles to four different levels of opacity.  They should be about 25% opaque, 50% opaque, 75% opaque and close to 100% opaque.

4.    Scratch another bottle to 100% opacity.

5.    You now have 7 bottles:-

a.    2 x unscratched bottles
b.    1x 25% scratched bottle
c.    1x 50% scratched bottle
d.    1x 75% scratched bottle
e.    2 x 100% scratched bottles

6.    Fill all 7 bottles with an equal amount of tap water (about 1 Liter).

7.    Next, mix each bottle with 3gm of E.Coli culture and close the bottle cap.

8.    Place 5 of the bottles (4 scratched (from step 3) and 1 unscratched) under sunlight for 6 hours.

9.    Place the other 2 bottles (1 x 100% scratched and 1 unscratched) in the shade for 6 hours

10.    6 hours later (make sure there is continuous exposure of sunlight during the 6 hours period), collect all the bottles and take a sample from each bottle and put 5ml of each sample onto an agar plate. 

11.    Place the samples into an incubator set at 38 degrees Celsius for 24 hours.

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Complexity level:
Project cost ($):
Time required:
6 hours for sunlight exposure, 24 hours incubation period
Material availability:
Tap water or rain water are acceptable sources of water. Agar plates are available at school biology laboratories.
Safety concerns:

Students must take personal safety steps by wearing gloves, mask and goggles before starting the experiment. Disinfectants should be available for standby in case of bacterial contamination. Teachers should be over watching students at all times during the experiment in case of any accidents. Students with respiratory problems should refrain from doing this experiment.