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Activity: Observing Bacteria Cultures in Yogurt (2)

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Materials:
  • Compound light microscope, like THIS or any of THESE.
  • slides
  • coverslips
  • dropper
  • plain yogurt
  • collection vial or cup
  • toothpick

Procedure:

  • Find a small vial and clean it thoroughly. If a collection vial is unavailable, a plastic cup will do. Make sure ALL soap is completely rinsed off.
  • Put a small amount of yogurt in the container, and put it aside in a dark, relatively warm area. Leave undisturbed for at least 24 hours.
  • After the time has past, take a small sample with a toothpick and place on a slide. If the sample seems too thick, dilute with a drop of water. Next, place a cover slip on top.
  • First observe the bacteria at low power 100X to find a good place to start looking. The diaphragm setting should be very low (small) because these bacteria are nearly transparent.
  • Switch into the highest power to identify the bacteria according to arrangement.

Bacteria are classified as follows:
First observe the way the bacteria are arranged:

  • paired = diploe
  • chained = streptose
  • clusters = staphyle

Next observe the shape of the bacteria:

  • round = coccus
  • rod = bacillus
  • spiral = spirillum

From here you can identify any bacteria you might find. For example, a common inhabitant of yogurt is a paired, round bacteria or diplococcus.
Be sure to clean the collection vials and slides thoroughly after usage.
Questions:

  1. How many kinds of bacteria could you find?
  2. What are doctors looking for with strep throat?

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