Multiplying Microbes

Category: Learning Cycle / Experiment

Learning Objectives:

1. Students will culture bacteria using agar plates.
2. Students will design and conduct an experiment.
3. Students will identify and describe some environmental conditions that effect bacteria growth

Materials, equipment and / or facilities:

• 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
• basic glassware normal for labs
• Matches
• Hot plates
• Acid rain mixture (add 10 ml conc. HCl to 1 liter of water)
• nitrogen fertilizer
• phosphate soaps
• 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.

Sequence and duration of each part of lesson:

Exploration stage
Part 1 (45 min)

1. Place students in pairs, and give each 3-4 petri dishes with 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.):
   1. How does environmental changes effect the growth of bacteria?
   2. What kind of data do you have to support your findings?
      Follow up: Does anyone have data that would support or refute these findings?
   3. Was your prediction correct?
   4. What do you like or dislike about each others' experiments?
   5. 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.

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   Copyright 1996. Utah State Office of Education, Curriculum Section, 250 East 500 South, Salt Lake City, Utah, 84111.

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