List of experiments





Ecological Succession
Environmental Microbiology
Experiments for middle and high school

Christine L. Case
Skyline College


  1. Determine pH.
  2. Graph pH vs. time.
  3. Provide the rationale for ecological succession.

Background   An assemblage of species living close enough together for potential interaction is called a community. Members of a community interact in a variety of ways. The best known are the relationships between predator and prey. More subtle interactions can be observed in a microbial community. As one organism grows, it changes the environment through production of acids or antibiotics which inhibit organisms that might compete for food. This is called competitive exclusion. Ecological communities change with time in a process called ecological succession. When one organism uses up its substrate of a limiting nutrient or is inhibited by its own end products, other organisms are able to grow. In this experiment, ecological succession will be examined in a microbial community.

Table 1.   Chemical composition of milk from selected mammals, in percent
Source Water Protein Fat Lactose Ash
Cow   87 3.5 3.9 4.9 0.7
Goat  88 3.5 4.3 4.3 0.9
Sheep   82 5.8 6.5 4.8 0.9
Buffalo 83 3.6 7.4 5.5 0.8
Reindeer   63 10.3 22.5 2.5 1.4
Whale   50 0 50 0 0
Human   87 1-2 3.5 6.8 0.2


Materials   Milk
Test tubes or beakers or glasses
pH paper



  1. Label test tubes or beakers. Put 10 mL milk in each tube or 250 mL milk in each beaker. Determine the starting pH of the milk. Add a pinch of soil and incubate the milk and observe for changes.
  2. As bacteria grow they may:
    Ferment lactose to produce lactic acid (curdling and decrease in pH).
    Hydrolyze the proteins. (Milk is cloudy due to insoluble proteins; as bacteria digest the proteins to form soluble amino acids the milk clears.)
    Some fermentation and hydrolysis products are alkaline which will increase the pH.
    When the pH returns to near 7, fungi are able to grow.
  3. Record changes in pH and appearance daily.
  4. Graph your data with pH on the Y-axis and time on the X-axis.


  1. What chemical change caused the pH to decrease?
  2. Why did the pH increase after the initial decrease?

Additional activities   Incubate milk at different temperatures to show the value of refrigeration and the danger of storing foods in a warm room

Source   A. Gillen et al, Milk Spoilage Poster, American Biology Teacher 50(5): 312-313, May 1988.