Multiplying Microbes

Standard II

Students will understand that the features of Earth’s evolving environment affect living systems, and that life on Earth is unique in the solar system.

Objective 02

Analyze how ecosystems differ from each other due to abiotic and biotic factors.

Indicator f

Plan and conduct an experiment to investigate how abiotic factors influence organisms and how organisms influence the physical environment.

Intended Learning Outcomes:

  1. Use Science Process and Thinking Skills
    1. Observe objects, events and patterns and record both qualitative and quantitative information.
    2. Use comparisons to help understand observations and phenomena.
    3. Evaluate, sort, and sequence data according to given criteria.
    4. Select and use appropriate technological instruments to collect and analyze data.
    5. Plan and conduct experiments in which students may:
      • Identify a problem.
      • Formulate research questions and hypotheses.
      • Predict results of investigations based upon prior data.
      • Identify variables and describe the relationships between them.
      • Plan procedures to control independent variables.
      • Collect data on the dependent variable(s).
      • Select the appropriate format (e.g., graph, chart, diagram) and use it to summarize the data obtained.
      • Analyze data, check it for accuracy and construct reasonable conclusions.
      • Prepare written and oral reports of investigations.
    6. Develop and use classification systems.
    7. Construct models, simulations and metaphors to describe and explain natural phenomena.
    8. Use mathematics as a precise method for showing relationships.
    9. Form alternative hypotheses to explain a problem.
Summary: Students learn techniques of culturing bacteria, and then they investigate environmental conditions that affect bacterial growth.

Materials:

  • 60 petri dishes
  • Nutrient agar medium to pour into petri dishes (can be ordered from any biological supply catalog, and follow directions for preparation)
  • Cotton swabs
  • UV light source
  • Thermometers
  • Glassware
  • Matches
  • Hot plates
  • Acid rain mixture (add 10 ml conc. HCl to 1 liter of water)
  • Nitrogen fertilizer
  • Phosphate soap
  • Ice
  • Other possible variables found in nature as seen necessary by teacher

Special Safety Considerations:

  • Caution students to keep lids on streaked petri dishes at all times. Some of this growth may be harmful.
  • When students finish with their petri dishes, collect and destroy them.
  • Follow chemical safety procedures when dealing with the acid rain mixture.
  • Be careful with all glassware

Sequence and duration of each part of lesson:

Exploration stage
Part 1 (45 min)

  1. Provide each group of students with 3-4 petri dishes containing agar.
  2. Model to students the proper technique for streaking a petri dish with a cotton swab.
    Instructions: After you have swabbed an area to be cultured, gently touch cotton swab to the agar and weave in a snakelike pattern from top to bottom. Then, rotate the dish 90 degrees and do the same snakelike pattern from top to bottom.
  3. Have students streak all of their dishes after wiping bacteria off of places in the room. (Throw away each cotton swab after one use).
  4. Students should put the lid on the petri dish immediately following the streaking. It should be turned upside down on the lid (so the agar is upside down), labeled, and stored in a warm place for 2 days.

Part 2 (45 min.)

  1. After 2 days have the students retrieve their petri dishes, and observe what bacterial growth looks like.
  2. Now ask the students, "What conditions were necessary to get the bacteria to grow." (You may want to write answers on the board)
  3. Ask the students to predict some of the environmental factors that either promote or inhibit the growth of bacteria. Then direct their attention to the materials available for them.
  4. Ask them to use their cultured bacteria to test some of the environmental factors that they predicted would affect the growth of bacteria.

    Specific Instructions:
    a. Predict how a specified environmental factor will affect bacterial growth.
    b. Design and conduct an experiment to test your prediction.
    c. Keep a detailed lab notebook or notes.
    d. Record data in drawings and graphs.
    e. Prepare to give an oral presentation on your experiment in one week.
    f. Use the available materials for your experiment.

Part 3 (10 minutes every day for 1 week)

Let students make observations and measurements on a daily basis.

Part 4 (45 minutes)

  1. Students should make final measurements and collect final data.
  2. They should clean up their entire experiment (unless they need to use it for their oral presentation).
  3. They should analyze their data, write their conclusions, and prepare their presentation. (The teacher may want to schedule the oral presentations at this time.)

Concept Invention stage
Part 5 (30 min)

In this part of the lesson, the teacher needs to direct a discussion from the groups. The teacher's questions should bring out data and conclusions from the groups and allow all answers that have good scientific proof (data supports the conclusions). Follow the order of questions below. (Make sure everyone participates.):

  • How does environmental changes effect the growth of bacteria?
  • What kind of data do you have to support your findings?
    Follow up: Does anyone have data that would support or refute these findings?
  • Was your prediction correct?
  • What do you like or dislike about each others' experiments?
  • If you were to do this experiment again, what would you do differently? Why?

Application stage
Part 6

Have students use their data and conclusions to write a half page paper on this question: "If the environmental change that your group tested were to actually happen for the next five years, explain what our biosphere would look (be) like."

Evaluation:
Possibilities for evaluation and assessment:

  • Oral presentations can be judged by the students.
  • Oral presentations should be graded by the teacher.
  • Have students research in the library, and use their data to write about some bacterial problem throughout history.
  • Have a contest between the students to try to preserve milk the longest.
  • Check for quality of experimental procedures.