Tools and Equipment of Science
Sessions 2-4: Follow-up Observations
Using a Petri dish as a template, each student should draw 3 circles on a sheet of plain paper. (Each group will use four sheets of paper, one sheet for each sample location.) The circles should be labeled “Day 1,” “Day 2” and “Day 3.” One student should prepare an extra sheet and label it “control.”
Instruct group members to draw what they observe on one of the inoculated Petri dishes, with each member observing a different sample location dish. Students can take turns drawing what they see on the control dish. If possible, have them observe microbe colonies through the Petri Dish lids, using a microscope on low power.
Remember, students are not to open the Petri dishes.
Student observations and drawings should be carried out for 1 to 3 consecutive days. Have students describe changes inside the Petri dishes over time, including the color, size, shape and apparent texture of the colonies. Help students to recognize the fuzzy, felt-like appearance of molds (fungi) on their plates.
Ask the students, How many different kinds of organisms can you distinguish on your Petri dishes? What criteria are you using to arrive at your number? It may be useful for students to examine their samples with a hand lens, as this will provide better detail.
It is not necessary for students to identify which of the four major groups of microbes are present. However, ask them which groups are most likely be visible on the agar. Answers should include bacteria and fungi. It would be rare for protists to appear on the agar. Virus particles could be on the dish, but they would not be visible.
After three days, have students count and measure the diameter of the colonies on their plates. Let them decide which sample location produced the most microbes. You may even want them to add a third column to their table, labeled “Actual Results.” In this column, students can record the results for each sample location and explain if they predicted correctly.
Each group should share its results with the class.
- Moreno, N., Tharp, B., Erdmann, D., Rahmati Clayton, S., Denk, J. (2007). The Science of Microbes Teacher’s Guide. Houston, TX: BioEd.
- Agar plate. National Oceanic and Atmospheric Administration. Retrieved 12-15-2009 from http://en.wikipedia.org/wiki/File:Agar_plate_with_colonies.jpg
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