Effect of Temperature, Light and Disinfectants on Bacterial Growth

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Author: Janice VanCleave

Bacteria can be found everywhere. A single bacterium cannot be seen but, if provided with the right conditions, will multiply into a visible colony, or group, of bacteria cells.

In this project, you will provide a nutritious diet for bacteria so that they can be grown for observation. The semisterile conditions of this project will allow you to test the effect of temperature, light, and disinfectants on bacterial growth.

Getting Started

Purpose: To determine how to culture bacterial growths.


  • 24 small baby food jars with lids
  • dishwashing liquid
  • water
  • paper towels
  • knife
  • small potato weighing about 6 ounces (170 g)
  • small saucepan
  • stove
  • 0.25 ounce (7 g) of unflavored gelatin
  • 1-cup (250-ml) distilled water
  • spoon
  • 1 beef bouillon cube
  • cookie sheet
  • oven
  • marking pen
  • masking tape
  • soap


  1. Wash the jars and lids with dishwashing liquid and rinse with water. Turn them upside down on paper towels to drain.
  2. Use the knife to peel and cut the potato into small pieces.
  3. Place the potato pieces into the saucepan with 2 cups (500 ml) of water.
  4. Place the saucepan on the stove and boil the potato pieces until they are well done. Drain them and save the liquid potato broth.
  5. Sprinkle the gelatin over the surface of 1 cup (250 ml) of distilled water. Allow to stand for two minutes and then stir.
  6. Add the gelatin and bouillon cube to the potato broth in the saucepan.
  7. Cook over medium heat, stirring constantly, until the bouillon cube and all of the gelatin dissolve.
  8. Pour the hot gelatin mixture into the four jars in equal amounts.
  9. Quickly secure the lids on each jar.
  10. Place the jars on the cookie sheet and bake them in the oven at 250°F (121°C) for one hour.
  11. Allow the jars to cool before removing them from the oven. Place 20 jars in a refrigerator for use in later experiments.
  12. Allow the gelatin to congeal in the remaining four jars, then lift the lid of one jar. Keep the lid over the jar as you press several of your fingertips, one at a time, against the gelatin. Do not cut into the gel.
  13. Secure the lid on the jar. Use the marking pen and masking tape to number the jar 1 and label it "Fingertips."
  14. Wash your hands with soap and dry them with the paper towels.
  15. Rub your fingertips across a well-traveled area of a tile floor.
  16. Lift the lid of the second jar and press several fingertips against the gelatin. Secure the lid. Number the jar 2 and label it "Floor."
  17. Repeat the procedure (steps 14 through 16) to collect a sample from a doorknob. Number this jar 3 and label it "Doorknob."
  18. Do not open the last jar. Label it 4 and "Control."
  19. Place the jars in a dark, warm place, such as a closet containing a water heater, for two to four days (see Figure 19.1).


Spots of growth can be seen on all of the test surfaces except that of the control jar. If growths are seen in the control jar, prepare new jars, bake the jars for a longer time period, and then repeat the experiment

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