2. Life Traps

Life trapsLife trapsAs part of recognizing the characteristics of life that all organisms share, students grow microbes on nutrient agar plates. Students swipe surfaces with a sterile Q tip swab and seed plates resulting in a wide range of colorful and prolific bacteria and fungi colonies. Other plates may be simply opened to the air to catch life floating in the air. Through these experiences, students learn that all living things, even those so small and invisible as to be floating in the air, grow and reproduce when provided with the proper nutrients and water. Teachable moments abound since the “dirtiest places”, like the toilet rim, often result in the least bacterial growth while presumably “clean” places, like the surface of your skin, have the most. A fun extension of this activity (see the Going Further section) is to start a sourdough culture from wild yeast in the air and make sourdough bread.

Can grow microbes on nutrient agar plates.
Can make observations and keep track of data over several days.
Can identify the typical growth patterns of bacteria versus fungi.
Can begin to recognize the diversity of microbrial life in the local environment.
Can explain that all living things will grow and reproduce when provided with the proper nutrients and environmental conditions.


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2. Life Traps - Logistics

30 min to introduce the activity and seed the plates
5-10 min to make observations every other day over the next 2 weeks

Individual or in pairs.

For approximately 50 plates you need:

  • 50 sterile disposable plastic 15 mm x 100 mm Petri dishes (can be purchased from Ward’s Natural Science, item number 18 V 7101, approximately $4 for a package of 20 dishes)
  • 15 g agar agar powder (a gelatin substitute made from red seaweed, available at most Asian grocery stores and health food stores, ideally purchase the unsweetened variety but pre-sweetened is OK as long as you ensure that you subtract out the weight of the sugar when measuring out 15 g of agar agar)
  • 2 beef bouillon cubes
  • 1 liter distilled water
  • stove and large pot for preparing nutrient agar and steam sterilizing the Q tips (better yet, if you have a pressure cooker, you will be far more assured of initially sterile conditions in your agar plates)
  • 50 Q tips
  • paper towels
  • permanent markers for labeling plates
  • masking tape
  • bleach


2. Life Traps - Background

Teacher Background
When living things are provided with the proper nutrients, water, and environmental factors (temperature, humidity, etc.) they will grow and reproduce, often explosively and in surprising ways. To kids, microbes are abstract, invisible germs that mysteriously spread disease, but otherwise have little relevance to their daily lives. However, microbes in the form of bacteria, fungi and viruses are prolific and exist all around us and even inside us. Of the 100 trillion cells found within your skin, only 10 trillion of these (a measly 10%) are human cells! The rest are primarily bacteria living on the surface of your skin, around your eyes, mouth, reproductive organs, and digestive tract, comprising between 500-1,000 different species. Since bacterial cells are generally much smaller than human cells, a great many more bacteria fit into the borders of the human body than human cells.

Nutrient agar plates are a classic tool for culturing microbes in the laboratory. The agar plate provides all the basic organic molecules - water, carbohydrates, fats, and proteins - and will support the growth of many bacteria and fungi species, though by no means will all microbes find the appropriate nutrient and environmental conditions to grow on an agar plate. It is important for students to recognize that many organisms that arrive on the plate will not have the right conditions to grow, thus, an empty plate does not necessarily mean no life exists there – only that this method could not detect life.

Luckily, pathogenic bacteria find it difficult to grow on nutrient agar so it is reasonably safe to use in schools. However, do NOT open the dishes once they have been seeded. High levels of mold and bacterial spores can be released. When you are finished with the experiment, disinfect the plates before disposal.

It is difficult to positively identify specific species from so rough a measure, however, it is possible to determine certain things about the colonies that grow. Bacteria tend to form low-growing buttons or streaks that are glistening and smooth. On the other hand, fungi tend to form fuzzy, irregular patches or fern-like, thread-like patterns. Different varieties can be distinguished by their color and texture. Colonies that were collected on a Q-tip swab from a hard surface will begin their growth where the Q-tip touched the agar. Colonies from the air that landed on the agar while the lid was open will begin as a dot located on part of the plate not touched by the Q tip swab. Each dot represents a single spore that landed on the plate.

Student Prerequisites


2. Life Traps - Getting Ready

Getting Ready
To prepare agar plates:

  1. Thoroughly wash your hands with soap and water.
  2. Set the pot (or pressure cooker) on the stove.
  3. In the pot, mix together 1 liter of distilled water, 15 g of agar agar, and 2 beef bouillon cubes.
  4. Heat to boiling, stirring occasionally to fully dissolve the agar agar and bouillon.
  5. If using a regular pot, simmer on medium-low for at least 30 minutes to get as sterilize as possible. (It is impossible to completely sterilize a solution simply by boiling but it’s good enough for a rough classroom experiment such as this one – 25% of the plates I made were contaminated, but that also means more microbes for the students to observe). To ensure a sterile starting solution, use a pressure cooker and cook for 15-20 minutes at 15 psi.
  6. While the nutrient agar is boiling, clean your work surface with warm water, soap, and perhaps even an antiseptic cleaning solution.
  7. Open the packages of Petri dishes and line them up on your work surface. Keep the lids on!
  8. When the nutrient agar has finished cooking, open the lid of each Petri dish as little as possible while carefully pouring 15-20 ml of the solution into the dish – each dish should end up between a third to a half full. If possible, hold the lid directly above the dish while pouring to prevent air-borne particles from settling onto the surface of the agar. Replace the lid immediately. This homemade nutrient agar will contain flecks of protein and fat from the beef bouillon, it will not disturb the experiment but students should carefully note the location of these flecks so that they are not mistaken with microbe colonies.
  9. Once the nutrient agar has solidified (10-15 minutes) it will turn cloudy and opaque as opposed to translucent. At that time, turn the plates upside-down to prevent condensation from pooling on the surface of the agar.
  10. Store them stacked upside-down in the refrigerator in their original plastic sleeves until ready to use.

To sterilize Q-tips:

  1. Wrap 10 Q-tips in a paper towel. Make several packets of Q-tips, enough for 1 Q-tip per agar plate.
  2. Set a pot (or pressure cooker) on the stove with water and a steamer basket in place.
  3. Place the Q-tip packets into the steamer basket and boil the water.
  4. Steam the Q tips for at least 30 minutes. If using a pressure cooker, cook for 15-20 minutes at 15 psi.
  5. When the Q-tip packets finish steaming, place them into a new ziplock bag and seal them until ready to be used. While the exterior of the paper towels will be contaminated, the Q-tips inside should be relatively sterile.

2. Life Traps - Lesson Plan

Fish tank microbes: Fish tank microbes collected by Woody, 6th grader, February 2006.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.Air-borne microbes: Air-borne microbes collected by Irene Salter from her classroom with a 30 minute exposure.

Lesson Plan
Introducing the activity and seeding plates:

  1. Introduce the agar plates to your students. Discuss what ingredients you used (water, agar, and beef bouillon) and the reason for including each.
  2. Ask the students to guess what they think might happen if a microbe found its way onto the agar plate.
  3. Have students brainstorm about microbes they know already or have heard about – many of these will be disease related.
  4. 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.
  5. 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.
  6. Have students wash their hands thoroughly with warm water and soap.
  7. Each student should be given an agar plate and around 6 inches of masking tape.
  8. 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.
  9. 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.
  10. Students can now get a Q-tip (if they want one) and seed their plates.
  11. When students return, in their lab notebooks they should make their first observation, being sure to note the following information:
  12. 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?
  13. A drawing of the exact pattern drawn by the Q-tip on the surface of the agar.
  14. 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.
  15. The Petri dishes should be stacked on a countertop upside-down until the next time students make their next observation.

Further observations and discussions:

  1. 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.
  2. Use a hand lens to examine any new growth. Measure the diameter of any colonies. Note their color and texture.
  3. Describe any changes since the previous observation
  4. 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.
  5. 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.
  6. 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.

2. Life Traps - Assessments


  1. Collect students’ lab notebooks with their observations and conclusions.
  2. Revisit the characteristics of life list from the Is It Alive? activity. Revise the criteria as necessary.

Going Further

  1. Test the effects of various antiseptics. After growth has taken place, add a piece of filter paper soaked in an antiseptic cleaning agent (Lysol®, bleach, 409, rubbing alcohol, Neosporin®) to the plate. Be very careful when opening the agar plates. Wear a dust mask and stay in a well ventilated area since the high concentrations of spores can cause lung distress.
  2. Make new life traps and test the effect of different environmental factors. For example, with similarly seeded plates, place one at room temperature, one in a warm place like near the water heater, and one in the refrigerator. Or investigate the effect of sunlight versus darkness.
  3. Make new life traps and compare the effectiveness of various manipulations thought to disinfect surfaces. Compare plates seeded with unwashed versus washed hands. Compare a table top before and after cleaning.
  4. Trap wild yeast from the air and use it to make sourdough bread! A mixture of water and flour provides the nutrient base for the yeast to establish itself. There are hundreds of recipes and different ways to create a sourdough culture. Before store bought yeast, sourdough cultures were the primary means of leavening bread. Since each culture has a slightly different population of yeast, every culture will produce its own distinctive flavor. Starting a sourdough culture is very simple to do and extremely fun. The Exploratorium website provides one way to start a culture using a lump of dough. Other sourdough starters use a more liquid culture with a consistency more similar to pancake batter than bread dough. For instance, the How Things Work website describes the procedure for creating a wild yeast starter with this more liquid consistency. I started my own sourdough starter using a procedure found in the cookbook, The Cheeseboard Collective Works, published by my all time favorite Berkeley bakery and cheese shop, The Cheeseboard.

2. Life Traps - Sources

The inspiration for this lesson was Mission 11 from the Life in the Universe curriculum, published by the SETI Institute. The recipe for the nutrient agar came from Biology F.A.Q. at Flinn Scientific. Sterilization tips were found at Science-Projects.com and Wikipedia. Finally, many teaching tips were discovered from Leslie Hathaway’s Bacteria Gathering lesson plan.

Statistics on bacteria in the human body were taken from Wikipedia.

Grade 6
5. Organisms in ecosystems exchange energy and nutrients among themselves and with the environment. As a basis for understanding this concept:
e. Students know the number and types of organisms an ecosystem can support depends on the resources available and on abiotic factors, such as quantities of light and water, a range of temperatures, and soil composition.

Grade 7
Cell Biology
1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope.

Investigation and Experimentation
7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
a. Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
e. Communicate the steps and results from an investigation in written reports and oral presentations.