Lesson 2

Worms in a changing environment: How does high salt affect C. elegans?

Driving Question:
How do wild type and mutant nematodes respond when transferred to low and high salt plates?

By the end of lesson two we hope you will be able to:

Know how to transfer wild type and mutant worms to low and high salt plates by chunking
Make and record observations of worms 15 minutes after transferring them to the new plates
Identify similarities and differences in the response of the two worm strains to an environmental change after 15 minutes exposure

In this lesson, you will observe what happens when we change the environment of the wild type and mutant worms from a normal "low salt" environment to a challenging "high salt" environment. You will look for behavior changes in the worms, such as changes in movement, signs of eating, evidence of reproductions, and general vigor.

Moving the worms by chunking

How worms are grown in the lab: In the lab, worms live on round Petri dishes half filled with agar. The agar contains salt and nutrients for the E. coli bacteria which are food for the worms. The E. coli bacteria are spread out in a yellowish circle on the agar--at least until the worms eat it up. As the worms feast, the yellow circle goes away.

To transfer the worms, cut a small square out of the worm plate and flip it upside down onto a new plate with fresh food. Mark the "drop site" so you know where the worms started. The worms will move onto the new plate and start looking for food right away.

It is important to use sterile technique for this so that harmful bacteria and/or mold are not introduced onto the plate.

(The type of E. coli used as worm food are not harmful to humans.)

A video is worth a thousand words!

Ayah will show you how to chunk worms.

When you are finished, you have four plates:

1 plate of Wild Type (WT) worms on LOW salt

1 plate of Wild Type (WT) worms on HIGH salt

1 plate of Mutant (MT) worms on LOW salt

1 plate of Mutant (MT) worms on HIGH salt

We are now ready to start the experiment! Ayah has chunked the worms for us in the video. We will wait 15 minutes to make our first observations. We will make observations again after 24 and 48 hours to see how the worms are getting along.

What do you think will happen? When moving worms from low salt plates to low salt plates, we aren't really changing their environment. Would you expect to see a behavior change? What will happen when we move worms from a low salt to a high salt environment? Will there be a difference between the wild type and the mutants? Let's find out.

Worm Observation Data Table

Make a copy of the Worm Observation Data Table. You will need it when observing the worms.

You will be looking for signs of the following behaviors:

Movement: Worms have left the drop site, worm tracks are visible through the food and agar, worms have spread out over the plate.
Eating: Worms are in the food. Worms may have stopped moving at food. You may see movement in the body of an adult.
Reproduction: Eggs may be found outside of the drop site. There are more worms on the plate than at last observation. Many life stages visible.

You will also make comments as to the general state of the worms. Are the worms thriving or challenged? Are there any other behaviors to report? Describe any other observations.

Worm Observations -- 15 minutes after chunking

Now we will observe and compare the behavior of two types of worms in both the old and the new environments.
Watch the following videos and fill out the first row of your observation data table.

Wild Type worms on LOW salt

Wild Type worms on HIGH salt

Mutant worms on LOW salt

Mutant worms on HIGH salt

Lesson 2 Comprehension Questions

A. In which of the four videos did the worms seem most challenged--as in not moving, shriveled, or maybe even....dead?

B. What do you predict will happen to the worms in each situation after another 24 hours?

C. What environmental changes do you think nematodes experience throughout the day, and throughout the year?

Next Step: Go to Lesson 3

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