Dirt Life

Authors/Adaptors: Owen Solberg and Scott Fay

Overview: After an interest-generating discussion about "dirt" and microbes, students select and collect soil samples from a variety of locations (schoolyard, home, etc.). They make a dilution in sterile water, plate it on a nutrient agar Petri dish, and then observe it 1-7 days later.

Lesson Concepts:

  • Soil microbial biodiversity is everywhere, it varies with soil type and depth, and it is very complex.

  • Microbes can interact and compete much like plants and animals.

  • Skills in microbiological laboratory techniques (measurement, culturing, plating, and pipetting) permit the study of microbes.

Grade span: 5-10

Materials:

For the class:

  • Soil samples from 2 locations, with descriptions and photographs if students are unable to visit the locations.

Per student:

  • 1 tube sterile water (for dilution)

  • 1 empty Petri dish

  • 1 eyedropper

  • 1 sterile nutrient agar Petri dish (LB agar or soy triptic agar would do)

  • Dirt Life notebook insert (pdf)

Background:

Healthy soil has an incredibly rich microbial community, so much that the biomass of soil is mostly microbes — more than insects, worms, plants, etc. Soil microbes primarily act as decomposers, breaking down dead plant and animal parts so that the nutrients can be used again. But there is also a rich interaction among microbes and between microbes and living organisms. For instance, Arthrobotrys oligospora, the "Nematode-Trapping Fungus," is able to capture, kill, and digest nematodes (a true animal). This could be shown by way of a computer video. The ASM has good microvideos at http://www.microbelibrary.org/asmonly/Details.asp?id=1767. An interaction easier for the students to witness will be the inhibition of some microbial growth by the presence of another microbe. For instance, a fungus may be able to inhibit the growth of a nearby bacteria by releasing antibiotics. This could be observed on the Petri dishes, especially if they were viewed more than once.

Extensions:

  1. The goal of the exercise is for the students to practice making careful observations. To that end, the teaching should be as hands-off as possible, and the students should have as much time as possible to make their observations.

  2. Incubate culture for another 1-6 days.

  3. Examine Petri dish for evidence of microbial interaction. Students will probably be able to observe growth inhibition by one microbe to another.

Acknowledgements:

Adapted from http://www.cubist.com/docs/LP1_Microbial_Diversity.pdf

Procedure:

  1. Briefly introduce the idea of microbial diversity (5 min. max.) "There are many kinds of microscopic organisms, some helpful, some harmful, living nearly everywhere …." Show large Petri dishes with cultured handprints from washed and unwashed hands. This will give the students some sort of idea of what happens when you culture microbes on LB agar. Tell them that the plate provides nutrients so that many different kinds of microbes can grow. Explain that today we are going to study microbes that live in the soil. Most of these microbes are harmless. Many are beneficial to the environment; they decompose matter and recycle nutrients. You may want to use an example like a pile of dead leaves to touch on the idea of decomposition.

  2. To each group of four students give a "microbe field kit," each containing two LB agar plates, two small plastic bags (Ziplocs), two tubes with (sterile) water, two plastic spoons, two eyedroppers, and one plate spreader. The plates and tubes can have tape on them to make labeling easier, each set numbered "1" and "2."

  3. Instruct each group to form two pairs, pair #1 and pair #2. Each pair will take a Ziploc bag and plastic spoon from the kit.

  4. As a class, go to the sampling site and take samples (10 min. max.). Pair #1 samples from a location without much plant life. Pair #2 samples from a location where there is a lot of potentially decomposing material, like under a bush or some wild grasses. Show the students how to take a spoonful of soil and put it into the bag. After students have collected their soil samples, return to the classroom.

  5. Students observe the soils (5 min.). Now back in the classroom, the students should write their observations into their notebook. What color is the soil? Is it wet, or dry? Is it stuck together into a clump or is it loose? Do you see any plant parts? Do you see anything alive in the soil? What is different about the two types of soil that your group collected?

  6. Students inoculate the water (2 min.). Take a pinch of the soil and put it into a labeled tube. Model for the students how to open the tubes and take just a pinch of the soil with the spoon. Seal the tube and shake.

  7. Students spread the inoculum onto the plate (5 min.). Model for the students how to take a drop of water and spread it using the plate spreader. Remember to put soil #1 onto plate #1, #2 onto #2. They should label plates with last names and period.

  8. Students make hypotheses and record them (5 min.). Remind them of what they already know — that the soil is full of microbes. They should already have some idea of how the plates work from observing the handprint on the large culture plate.

  9. Collect the plates. Culture over the next few days at room temperature. Observe the plates daily. After you can see colonies, but before the colonies start to grow into each another, wrap the plates in Parafilm and refrigerate until the next session.

2nd session

  1. At the next session, after the colonies have grown up, return the students' plates (10 min.). Emphasize that they should not open their dishes. The dishes will smell bad if they open them. They should spend some time looking at the plates, and then they should draw the colonies they see on each plate. (Note: if plates have been in the refrigerator, try to take them out at least an hour ahead of time to minimize condensation)

  2. Students make written observations of their plates (10 min.). See the notebook insert. Go through each step with the students, especially English language learners.

  3. Students observe using dissecting scopes or hand lenses (5 min.). If students don't have scopes at their desks, they can move to benches with scopes already set up. Hand lenses (10x) should be sufficient to see the details of the fungal hyphae, but dissecting scopes are preferred because of the better light and the opportunity to practice microscopy.

  4. Students make drawings of two colonies that look different from one another (10 min.). There are two main kinds of organisms they will observe: bacteria and fungi. They are fairly easy to distinguish. Bacteria will form plain round smooth colonies. Fungi can have many different shapes, but generally are much more textured and more interesting to look at than bacteria.

  5. Wrap up the lesson (5 min.). Revisit their hypotheses. How well did your observations match what you expected? What other kinds of living things would you expect to see in the soil? [plant roots, worms, insect larvae (grubs), insects, moles, etc.] Explain how there is more mass of microbes in soil than all the other living things combined. Microbes are essential to the health of the soil. They are nature's recyclers and help rid the world of dead organisms. What would happen to the Earth if there were no decomposers?

 
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