Living in an Alkaline Environment

Part Two: Testing Bacterial Survival Over a pH Range of 7 - 10

In this activity, you will test bacterial survival over a pH range from 7 to 10. You will inoculate four plates with local environmental samples and then observe bacterial survival patterns over a 24 - 48 hour period. Before you begin, document your predicted results. Upon completion of this exercise, answer the questions at the end. You may write the answers on your own sheet of paper or download this page in the form of a Word (Microsoft Word 24kB Apr25 05) document.

Your Prediction
  • How will pH affect the growth of the soil bacteria in your sample
  • 4 prepared, sterile agar plates (labeled pH 7, 8, 9, 10)
  • Permanent marker or wax pencil
  • Sterile, cotton-tipped applicator (e.g., cotton swab)
  • Soil sample from local environment
  • Pipette
  • Tape to seal the plates
  1. To avoid accidentally contaminating a plate, read the procedure in Step 2 before inoculating your four plates.
  2. Gather the materials and inoculate the four plates, using the following procedure:
    • Set the four plates on the table in front of you.
    • Take the lid off one of the plates. Hold it pointed face down to avoid having airborne particles land on it and contaminate it. Do not set it on a table.
    • Shake the soil slurry and use a pipette to transfer about 1/4 ml (250 microliters)—or about 5 drops - of it to the plate.
    • Streak the plate in a zigzag fashion with a sterile cotton swab.
    • Replace the lid, tape it closed, and turn the plate upside-down.
    • Label the plate with the date, time, pH, and group name.
    • Repeat the procedure with the other three plates.
    • Stack the inoculated plates and place the stack where your teacher directs.
  3. Check for growth over the next 24-48 hours. DO NOT OPEN THE PLATES.
  4. After 24-48 hours, count the colonies on each plate. If there are a lot of colonies, count the number in a segment of the plate (e.g., a quarter of the plate) and use this value to estimate the total number. Record your observations.
  5. Dispose of the plates as your teacher directs.
  6. Make a graph that compares the number of colonies to pH.
  1. What is the optimum pH for the soil bacteria found in your local environment?

  2. What advantages might bacteria get from living in a high pH environment?

  3. What might be some of the disadvantages of living in a high pH environment?

  4. What can Mono Lake's alkaline-loving bacteria teach us about life on Earth?