Spores to the Extreme

MATERIALS

  • Ziploc® sandwich baggies or clear, uniformly sized containers with lids (baby food jars, small storage containers)
  • medium for microbe growth (damp unpreserved bread, natural cheese, agar)
  • microbes of your choice (dry baker's yeast is best, available in food stores)
  • thermometers (1 per group, as small as you can find)
  • microscope or hand lens (optional)
  • other experimental materials as needed; see Procedure

CONTEXT
In Weather's Outer Limits, students discovered how extreme the climatic conditions in the polar regions can be. In this activity, they'll create their own experiments to test the effect of extreme environmental conditions on life forms—specifically, on microbes, which endure some of our planet's most extreme conditions in the Arctic and Antarctic. Scientists look to these extremeophiles as they ponder how life could exist on Mars or other planets. This activity asks students to consider what conditions permit bacteria and other microbes to survive.

PREPARATION
The day before the activity, discuss the procedure with your students. After your teams have decided on an experiment, provide them with items from the materials list and any additional items they will require. Each team should have enough materials to prepare several sets of specimens, including a control set, as follows:

  • Place an equal amount of medium in each baggie or jar. Although microbes are naturally present, you may want to add yeast.
  • Set one jar or baggie aside to use as a control. Teams should then apply one desired variable to each of the other sets of specimens.
  • Close the baggies or jars and don't open them again.
  • When the experiment is over, place the unopened containers in a heavy-duty trash bag, sealed with a knot, and have an adult take the bag directly to an outside dumpster.

PROCEDURE

  1. Divide students into teams of three or four and challenge them to propose and design their own experiment to gauge how microbes can handle an extreme environment. Note: Try to refrain from directing their experiments too much. The essence of this activity lies in allowing students to design their own experiment. How, for example, would they answer the following questions?
  • Temperature: What are the hot and cold "outer limits" for microbes? What is their ideal temperature for growth? (Note: Teams pursuing temperature will want to use jars big enough to enclose thermometers.)
  • Sunlight: How do microbes fare in direct sunlight or under a plant light? In complete darkness?
  • Moisture: The Dry Valleys of Antarctica haven't seen rain in two million years, yet microbes live there. What would happen if the water in a microbe's environment evaporated? Can microbes, or any living thing, exist without any water at all?
  • Pollutants: Can smoke, such as incense, alter microbe growth?
  1. Ask teams to write a proposal for their experiment. It should include a hypothesis, prediction, materials, and procedure. Make sure teams have addressed and controlled variables that might affect results and that they have included a control sample of microbes in the procedure. The more data collected, the richer the experiment. For this reason, having two or three groups testing the same environment and sharing information is a good idea (as well as common scientific practice).
  1. To conduct their experiments, teams will need to prepare several microbe environments in the same way (see Preparation). Non—experimental variables-temperature, light, moisture, etc.—should remain the same for each one. (Note: The "controls" for each experiment should be placed in a warm, moist, dark environment. Visible growth may require a week.)
  1. As teams observe growth, they should draw or record
    the growth in a notebook and keep a record of temperature, water added, and other variables.
  1. Discuss teams' results. Did any microbes not grow at all? Did some take longer to become visible? Which ones grew the fastest? What appear to be the ideal conditions for microbes? The worst conditions? Where would those conditions be found on Earth?

ASSESSMENT
Though most bacteria, molds, and other microbes are harmless to humans, some can be deadly. Have students propose a plan for completely sterilizing their experiment containers. What conditions could make the environment too extreme for microbes to survive? How could they ensure that all the microbes will be eliminated? Discuss with the entire group the importance of lab safety when working with microbes.

EXTENSION
Many microbes and spores are present in the air and can be collected by leaving the medium open to the air for a few hours.

You can also obtain them by scraping residue from the bottoms of students' shoes. Observing safety issues, try repeating some teams' experiments with different microbes you've collected in this way. Their results may differ. Afterwards, use a weak chlorine solution to sterilize equipment you've used.

Courtesy NSF/National Science Foundation