Effect of Temperature, Light and Disinfectants on Bacterial Growth (page 2)

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Author: Janice VanCleave


Bacteria are very small, one-celled, microscopic organisms that are found in air, water, soil, and the bodies of other living organisms. The gelatin provides a culture medium (specially prepared nutritious substance) on which bacteria grow into a visible colony (group of cells). Baking the jars kills any pre-existing bacteria. The placement of samples of material onto the culture medium is called inoculation. The lid is held above each jar during inoculation to prevent particles from the air from falling into the container.

Conditions Affecting Bacterial Growth

Try New Approaches

  1. How does temperature affect bacterial growth? Repeat the experiment making two samples of each of the three inoculations. Add "Warm" and "Cold" to the labels. Place the warm samples in a dark, warm place (such as the water heater closet) and the cold samples in a dark, cool place (such as a refrigerator).
  2. Do bacteria grow better where it is light or dark? Repeat the original experiment making two samples of each of the three inoculations. Add "light" and "Dark" to the labels. Place the light samples in a warm place where an electric bulb can shine on them all the time. Be sure that the bulb is not so close that it changes the temperature of the jars. Place the dark samples in a dark, warm place.
  3. Is deodorant soap more bactericidal (able to kill bacteria) than pure soap? Repeat the original experiment making two samples from each of the three testing surfaces. Rub your hands across one of the surfaces and ask a helper to wash one of your hands with deodorant soap before making one inoculation. Have your second hand washed with pure soap before making the second set. Repeat the procedure for each testing surface.
  4. Conditions Affecting Bacterial Growth

  5. Which deodorant soap is the most bactericidal? Repeat the preceding experiment using different brands of deodorant soap. Science Fair Hint: Triclocarban is an antibacterial agent that inhibits the growth of bacteria. Discover whether all deodorant products contain triclocarban by checking the labels on different deodorant soap and antiperspirant products. Make and display a list of the active ingredients in each product.
  6. Do disinfectants kill bacteria? Repeat the original experiment inoculating as many culture jars as you have samples of disinfectants. Rinse the surface of each gel with a different disinfectant after each inoculation. Use two controls—one with no inoculation and the other with no inoculation but rinsed with distilled water. Science Fair Hint: Since culture jars are usually not allowed to be displayed for safety reasons, display sample jars without the bacterial cultures. Take photographs of the jars and their contents as the experiment progresses to represent the procedure and results.

Design Your Own Experiment

  1. Warm temperatures promote the growth of bacteria that can cause food to spoil. Milk contains some bacteria and will eventually spoil even when refrigerated. Determine how long it takes milk to spoil at different temperatures. Place closed containers of milk in areas of different temperatures, such as at room temperature, in a refrigerator, and near a heater or fireplace (see Figure 19.2). Spoilage of milk is evident by the presence of thick, white lumps and a sour smell.
  2. CAUTION: Do not taste the milk used in this experiment. It could make you sick.

  3. Are there bacteria in the air? Repeat the preceding experiment using different containers of milk but leaving the jars uncovered. Compare the time required for the milk to spoil in an open container and a closed container. Science Fair Hint: Display graphs of your results.
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