Plant Biology and Microbiology (Bacteria) – Week 2
In the first half of the lesson: students will examine the results of their plant growth and bacterial
sampling experiments from last week, seeing whether the results match their predictions.
In the second half of the lesson: students will learn about symbiotic interactions between plants (legumes)
and nitrogen-fixing beneficial bacterial.
Part 1: Analyzing Results from Last Week
Plant Biology Experiment:
Tutors should spend a few minutes reminding students of what they did last week in the plant bio part of
the lesson. Ideally, this should be done by asking the students questions:
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What did we study last week? Plants and bacteria.
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What did we learn about plants? Plants make the oxygen we breathe and the sugar we eat. They
grow best when they have light, water, nutrients, and no pollution in their environment.
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What did we do with the plants? Tried growing them under different conditions to see how light,
water, and pollution affect their growth.
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What predictions did we make? This is a good time to get out the worksheet from last week so
students can remember their exact predictions.
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Were our predictions right? Students will now check their plates from last week to see if their
hypotheses were correct.
Procedure:
1. For each plate, ask the students to explain the results. Compare the plants in the different
treatments to the control plants. Have students identify how many seeds sprouted, how big the
seeds sprouted, their color plants.
2. Do the results match the predictions? If they don’t, why might that be? Try to get students to
come up with an explanation for the unexpected results.
Expected Results:
 Control/normal: plants sprout, developing roots and dark green leaves.
 No water: seeds don’t sprout or do anything.
 No light: plants sprout and grow, but do not look the same as control plants. They have very long
stems and are pale green. This is because the plants “think” they are underground, and are
trying to elongate as much as possible to emerge from the soil and find the light.
 Pollution (salt water): plants don’t sprout. Depending on salt concentration, they may sprout
some, but should be more stunted or have lower germination rate.
3. Have students write or draw a picture of their results on the worksheet from last week.
Bacterial Sampling Experiment:
Briefly remind students of the bacteria part of the lesson from last week. They will likely remember that
they got to collect samples, so try to focus on the purpose of sample collecting (to see how many bacteria
were in different places) and the predictions they made.
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What did we learn about bacteria last week? Bacteria are tiny microbes found in many places
around us. Some bacteria make us sick, but other bacteria are harmless, or even beneficial.
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What experiment did we start last week? Sampling bacteria from different places around the
classroom and garden.
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What was the purpose of the samples? To see how many bacteria are growing in different places.
We streaked the bacteria out on plates so that they could multiply and form colonies. Individual
bacteria are microscopic, but colonies contain ~1 million bacteria each, therefore we can see
them by eye.
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What predictions did we make? Try to get students to remember what places they sampled and
how many bacteria they expected to find there, using the worksheet to remind them if necessary.
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Were our predictions right? Students will now check their plates from last week to see if their
hypotheses were correct.
Procedure:
NOTE: Plates are sealed with tape and must be kept closed during analysis. The great majority of
microbes are harmless, but some could be “nasty” bugs, and opening the plates could release large
amounts of bacteria or fungal spores.
1. For each plate, ask the students to explain the results. Remind them that we can tell how many
bacteria we put on the plate by counting the number of colonies (one colony = one original
bacterium). Encourage them to look at different aspects of the colonies:
o How many colonies are on each plate? Compare to the control and the other plates.
o Do they all look the same? Consider size, color, shape, texture (gooey/dry).
o If there are different types of colonies, are they found on different plates? On one plate?
o Do the colonies all look like bacteria, or could some be fungi? Bacteria tend to form
small buttons or streaks that are shiny. Fungi tend to form fuzzy, more irregular colonies
or thread-like patterns. Explain to students that fungi are another type of microorganism.
o Do the results match the predictions? Were there places with more bacteria than
expected? Fewer bacteria than expected? Discuss whether places that we think of as
“clean” do or don’t have fewer bacteria than “dirty” places, like the garden.
Expected results: The type and number of bacteria varies with the location sampled. It is often
possible to see at least two different types of colonies on a single plate (as distinguished by
different colors/shapes/etc.). Plates left for more than two days tend to have fungi as well, which
can be recognized by their fuzzy/filamentous appearance.
2. Have students write or draw a picture of their results on the worksheet from last week. Students
can examine their colonies under the microscope and draw pictures of what they see there.
Part 2: Plant-Bacteria Symbiosis
Students will learn about Rhizobia, symbiotic bacteria that infect the roots of legume plants and provide
the plants with nitrogen.
They will examine nodules (symbiotic structures) on the roots of plants, and will plant their own pea
seeds to take home, dipping them in Rhizobium to help them grow.
Discussion
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Are there bacteria that make plants sick? Yes, bacteria can infect plants, though these are
different bacteria than the ones that infect people. If you see a plant with black spots or yellowed
leaves, it might be infected with bacteria.
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Are there “friendly” bacteria that help plants grow well? Special bacteria (called Rhizobium)
grow inside the roots of legumes, such as peas, beans, and alfalfa. In exchange for sugar from the
plant, the bacteria fix nitrogen, taking it out of the air and converting it to a form the plant can
use. This exchange benefits both the plant and the bacteria.
Root Nodules from Wild Clover:
Each group (supplies permitting) will receive a wild clover plant dug up from the Stanford campus. The
clover plant will have its roots washed off and exposed, and will be in water to prevent drying.
1. Spread the plant out on a plate and have students examine the roots. There should be nodules
(small bump-like symbiotic structures, ~0.5-1mm in length) growing on the roots.
2. Help students identify nodules, and count how many are on the roots.
3. What color are the nodules? Older nodules turn green, while young, healthy nodules are pink.
Students can break open the nodules to see the pink inside. (The pink is from a substance that
helps bacteria get oxygen, similar to the oxygen carriers found in our blood.)
Grow Your Own Pea Plant:
Each student will get to plant and take home his/her own pea plant. Students will dip pea seeds in
Rhizobium (a commercial inoculum preparation, sold for gardening), so the pea plants will grow better
with the help of the bacteria.
Materials:
Solo sundae cups or other plastic cups
Potting soil
Fertilizer pellets
Pea seeds
Water
Procedure:
1. Have students fill their cups with soil (using the cup to scoop soil from a larger bag is easiest and
least messy).
2. Pour water on the soil to make it damp, but not soggy. If there is too much water, pour some off
into the sink or a spare cup.
3. Take two pea seeds per student, dip them in water so they are damp, and place them in the plastic
Ziploc bag full of bacterial inoculant. Close the bag. Have students gently squish the
peas/inoculant around until the peas are coated.
4. Have each student poke two holes ~1 inch deep in the soil in the cup. The tutor (wearing gloves)
will place a pea seed into each hole. Students should push the dirt around the pea to cover it up,
and put the dome lid on the cup when they are done.
Instructions for sprouting and growing the pea plants at home: Put the plant in a sunny location, such as a
windowsill. Check the soil regularly, and keep it damp but not soggy. Once the plant sprouts and gets
larger, it should be transferred to a bigger pot or planted in the soil outside.
Additional take-home activity: We can’t make yogurt with the kids during Science Bus because we don’t
have the right facilities (and because it takes ~8 hours), but this is a fun and easy “practical microbiology”
project they can do at home. I will have copies of a simple yogurt-making recipe/protocol (attached) that
the kids can take home and try with their parents if they’re interested. I use this recipe often at home, and
it is fun and fairly foolproof.
Sources:
http://www.bio.net/bionet/mm/methods/2001-October/090763.html
http://csip.cornell.edu/Curriculum_Resources/CSIP/Mendell/Mendell.html
http://www.mysciencebox.org/book/export/html/439
Microbiology in the Kitchen: Make Your Own Yogurt!
Yogurt is made from milk by beneficial bacteria, primarily L. bulgaricus and S. thermophilus, in a process
called fermentation. You can easily make your own yogurt at home by following the recipe below. All
you need is milk and a starter culture (a few spoonfuls of yogurt you’ve bought from the grocery store).
The bacteria in your starter culture will grow rapidly, converting the milk into yogurt in ~8 hours.
Once you have made the yogurt, you can flavor it however you want by adding fruit, jam, honey, nuts,
or anything else you like!
Ingredients:
1 quart milk (skim, low-fat, or whole)
2 Tbsp. plain yogurt with active cultures
Directions:
Bring the milk to boil in a heavy pan. As soon as it starts to rise, turn the heat off and let the milk cool in
a pitcher or large measuring cup until it is just warm (98-110 degrees F).
Meanwhile, put the yogurt in a large ceramic bowl. Beat it until it is creamy. When the milk has reached
the correct “warm” temperature, add it very slowly to the yogurt, a little at a time, stirring as you go.
(Any film that has formed at the stop may be stirred in.) When all the milk has been added, cover the
bowl with plastic wrap and leave it in a warm place, where the temperature is around 85-98 degrees F.
You can also wrap the bowl in a blanket , or put it inside the oven if there is a pilot light to keep it warm.
In 6 to 8 hours the yogurt should set up. When refrigerated, it will last 3 to 4 days.
Variations:
Greek Yogurt: you can make the yogurt thicker by straining it with tripled cheesecloth (or a towel or
cloth napkin). Use the cloth to line a colander or sieve, pour the yogurt into the middle, and let drain
over the sink until the desired consistency is reached.
Suggested flavorings: fresh figs and honey, Nutella, fresh strawberries, pomegranate seeds, almond
extract.
Sources:
Madhur Jaffrey’s World Vegetarian Cookbook
http://nchfp.uga.edu/publications/nchfp/factsheets/yogurt.html