Five to six one hour classes (If you are on a block schedule, samples MUST be checked after 24 hours but, final observations can wait for 48 hours). Student pages have been written for 135 minute block classes.
TIME FRAME BREAKDOWN:
One introductory period (explaining scenario, discussing bacterial morphology and arrangement, planning experimental designs).
One period for an explanation of aseptic technique and for streaking cultures onto nutrient agar.
One period for gram staining, inoculating broth tubes, and incubating cultures at varying temperatures.
One period for observing growth, documenting results, and sharing results with the class.
One period for discussion of extreme environments and microbial populations.
Engagement and Exploration (Student Inquiry Activity
Present a letter to your students from a local laboratory (see "Student" pages) explaining that their laboratory technician has mixed up the labels on tubes of bacteria and they no longer know the optimal growth temperatures for these cultures.
Tell the students that the laboratory wants them to determine the growth temperature ranges for these bacteria and the optimal temperature for each culture.
Lead the students in the following discussion: How can we determine which cultures must be grown at which temperature? (Each culture must be grown in a variety of temperatures). How will we monitor growth? (There are several ways, including spectrophotometry. The easiest way is to use a scale of known substances with differing turbidities. A culture in which no growth appears might be transparent like water. A culture in which the bacterial growth is heavy might be compared to milk, an opaque liquid). Which temperatures should be tested? (Temperatures tested should range from freezers [0oC] to hot water baths [45oC, 55oC, 80oC]). Ask students, working with their lab partners, to design an experiment that would determine the temperature ranges and optimal growth temperature for each of the cultures. (These suggestions should include growing the cultures at different temperatures and monitoring the growth).
Elaboration (Polar Applications)
Discuss the different experimental designs and guide the students to a design that actually tests growth at various temperatures. Students will probably suggest that organisms should be incubated at room temperature, an oven (or some such heating device), the freezer, the refrigerator, etc. (You will actually use a freezer, refrigerator, hot water baths, incubator, and room temperature).
Have students read through the procedure and answer the accompanying questions in their lab notebooks.
Ask students to determine their negative control. (This should be an uninoculated tube of sterile media). Positive controls are not used because of the slow growth and media requirements of the psychrophilic organisms.
Define growth. What should the students observe when a culture is in logarithmic growth phase? (Culture tubes should be cloudy with growth) How will we measure this? (The use of a spectrophotometer is best but, it will be obvious to the students if their cultures grew or not. It is not necessary to actually quantify this observation with a spectrophotometric reading (see "Explanation" section above).
Were the cultures pure? How did we determine this? (Only one colony type grew on the petri plates and only one type of bacteria appeared on our gram stain).
What other factors affect growth of microbial communities? (Nutrient availability, amount of sunlight and pressure, concentration of metals, presence of pollutants, type of soil, stagnant vs. moving water, etc.). Discuss other variables that might be interesting to test.
Exchange (Students Draw Conclusions)
Discuss what we know about the bacterial communities in Antarctic Dry Valley lakes and in the hot springs of Yellowstone National Park. Ask students to compare the temperatures that might be encountered. Have they ever seen people bathing in hot springs? Show photographs of Antarctica, Yellowstone National Park, and other appropriate places. Ask students to describe what they see, feel, hear, etc.
Evaluation (Assessing Student Performance)
Each student is required to keep a lab notebook. This should be graded for completeness and accuracy (eg. temperature readings). Each student is required to compose a business letter suggesting possible labeling for the cultures tested. Allow them to arrange the bacterial cultures (designated only with single number) in a meaningful way. This letter should be in the correct format for a business letter and should address all aspects of the laboratory activity without repeating entirely the contents of the lab notebook.