2. Life Traps - Lesson Plan
|Fish tank microbes: Fish tank microbes collected by Woody, 6th grader, February 2006.
||Air-borne microbes: Air-borne microbes collected by Irene Salter from her classroom with a 30 minute exposure.
Introducing the activity and seeding plates:
- Introduce the agar plates to your students. Discuss what ingredients you used (water, agar, and beef bouillon) and the reason for including each.
- Ask the students to guess what they think might happen if a microbe found its way onto the agar plate.
- Have students brainstorm about microbes they know already or have heard about – many of these will be disease related.
- Discuss places where students believe microbes will be found. If nobody brings up the air, ask students whether they think microbes are found in the air. If nobody brings up their bodies, ask students whether they think microbes are found on or inside their bodies.
- Describe the procedures that must be used to safely trap life in the Petri dishes:
- Hands must be thoroughly washed with water and soap.
- Plates can be seeded by taking a dry, sterile Q-tip and rubbing it on a test surface or dipping it into a test liquid, and then very gently rubbing it on the surface of the agar. Do not gouge the surface of the agar. In order to tell the difference between microbes that were intentionally placed on the agar with the Q-tip and ones that fell into the plate from the air, the Q-tip should be swiped across the agar in a distinctive pattern – a zig-zag, your initials, a smiley face, etc.
- Plates can also be seeded by leaving them open to the air for 20 minutes in a specific location.
- Agar plates should be opened only as long as necessary to seed the plates.
- Once plates have been seeded and sealed, they should not be opened again.
- Do not collect microbes from other teachers’ classrooms.
- Do not collect microbes from other people’s bodies – you can collect from your own body or hair.
- Do not collect microbes from urine or feces.
- Have students wash their hands thoroughly with warm water and soap.
- Each student should be given an agar plate and around 6 inches of masking tape.
- Break off two 1.5 inch pieces of masking tape and use them to seal the plate shut on opposite sides. One side will make a hinge while the other side can be opened temporarily to seed the plate, then sealed again. With a permanent marker, label one side L for left and the other side R for right.
- The remaining 3 inches of tape can be stuck to the bottom of the plate and labeled with the student’s name and source of his or her sample.
- Students can now get a Q-tip (if they want one) and seed their plates.
- When students return, in their lab notebooks they should make their first observation, being sure to note the following information:
- A detailed description of how the sample was collected. Be as specific as possible. Which button on the telephone was rubbed, or was it the mouthpiece? How hard did you rub the Q-tip on the surface? Exactly where was the plate left open to the air and for how long? Was there a window open nearby? What else was nearby?
- A drawing of the exact pattern drawn by the Q-tip on the surface of the agar.
- A detailed labeled drawing of the agar plate oriented so left and right is correctly positioned. Any flecks of protein or fat droplets should be labeled and carefully described.
- The Petri dishes should be stacked on a countertop upside-down until the next time students make their next observation.
Further observations and discussions:
- Every other day, create a new labeled drawing of the agar plate. Try to draw any colonies as accurately as possible – the correct size, shape, and place on the plate.
- Use a hand lens to examine any new growth. Measure the diameter of any colonies. Note their color and texture.
- Describe any changes since the previous observation
- Once students start seeing colonies, tell your students how to distinguish between bacteria and fungal colonies. Tell them how to tell different microbes apart by their color and texture. Explain how to determine whether a colony was intentionally seeded by a Q tip or fell from the air.
- Discuss the different forms of life that appeared in this experiment. Specifically talk about how we know that these creatures are alive – they grow, they reproduce, they need nutrients and water, etc. Mention the possibility of things that grow yet aren’t alive such as crystals, magic egg creatures that expand to many times its original size in water, inflatable plastic toys, etc. Discuss how these growing things are different from living things and how one could tell the difference between them.
- There are many directions to take this discussion from here:
- Was it surprising to find microbes in the air? Using plates seeded from the air, calculate the number of microbes that landed per minute that the plate was open.
- Compare the number of microbes trapped in different locations. What may have caused more microbes to grow in some places compared to others?
- Do these life traps capture all the microbes in a given place? What factors might prevent a microbe from growing in these life traps? How would you modify the experiment to look for different microbes that didn’t grow in this experiment?
- How could a life trap be used to look for life on other planets such as Mars or Venus?
- When your investigations are over, the teacher should disinfect then dispose of the plates. One option is to prepare a bucket filled with one part bleach to 9 parts water. Submerge the plates in the bleach water to kill the cultures before disposing of them in the regular trash.