Activity: Observing Bacteria Cultures in Yogurt (2)

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


  • 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.

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

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