Chemotaxis on the Move – Active Learning Teaching Tool
Ann H Williams
Corresponding author. Mailing address: Biology Department, University of Tampa, Box 3F, 401 W. Kennedy Blvd., Tampa, FL 33606. Phone: (813) 257-3994. Fax: (813) 258-7496. E-mail:ahwilliams@ut.edu.
Collection date 2010.
INTRODUCTION
In Microbiology courses, concepts such as chemotaxis can be difficult to visualize for students. Described here is a short visual playacting activity where students simulateE.coli moving towards an attractant source using a biased random walk. This short interactive activity is performed in the lecture course of General Microbiology that contains mostly Biology major juniors or seniors prior to the lecture on the subject of chemotaxis and flagellar movements. It is utilized to help students (class of 30–40) understand and visualize the process of chemotaxis and the concepts of random walk, biased random walk, runs, tumbles and directed movement of flagella in response to attractants and repellents. First, the students learn that some bacteria can move in their environments and perform chemotaxis, directing their cell movement towards attractants and away from repellents. In a similar manner to our sense of smell, the bacteria sense attractants (like apple pie) and can move towards that source (1). The organisms produce flagella, and use cellular movements called runs and tumbles in a biased random walk (2,3). After this brief explanation to the students, the class performs the activity.
PROCEDURE
All students move outside and spread themselves out in a large open area, such as a large concrete deck area with grass boundaries. You will now instruct the students to act out a random walk.
Student instructions for random walk (Fig. 1)
FIGURE 1.
Demonstration of random walk.
Each star is a student and each will perform a random walk. The random walk for only three students is shown (dark black stars). Each student performed four runs (arrows) and four tumbles (circles) with the black circle tumble being the end location of the student. Each tumble can result in movement in a new direction and all runs are five steps, regardless of direction the student is facing. For all three students, the end tumble (black circle) is still not past the three-quarter line towards the apple pie.
Imagine that at one end of the space beyond the boundaries is a large apple pie or your favorite food (attractant). It was just cooked and you can smell it. To get yourself a slice of that apple pie, you could just walk right straight to the pie. However, you areE. coli and can only move in a random walk consisting of continual runs (∼ 1 sec) and tumbles (spin for ∼ 0.1 sec) (2).
Students: Start by running (walk straight for 5 steps) and then immediately tumbling (spin around). You will be facing in a new direction, so run again for 5 steps and then tumble and then run for 5 steps in a new direction. Try to get to the apple pie by doing this random walk.
Instructors: Start the students off by saying GO. Let them do this for a couple of minutes and then say STOP. Tell the students to look at where they are – how close are you to the apple pie? Most students are still randomly distributed throughout the space and even though they knew where they wanted to go (for the apple pie) they couldn’t get there by performing a random walk. Explain that this isE.coli with flagella (movement) but with no chemotaxis (sense and directed movement) (2). Ask the students if this is the best way to get to the apple pie or nutrients forE.coli. If not, what can be done without eliminating the run or tumble (you still have to do both)? Some students guess that you can extend the runs in the direction of the pie.E.coli and other bacteria can bias their random walk towards an attractant source or away from a repellent source (2). This is called the chemotaxis system – the ability to sense the attractant and direct your movements in that direction using both runs and tumbles (biased random walk) (1,2,3). We will demonstrate this now using the next activity.
Student instructions for biased random walk (Fig. 2)
FIGURE 2.
Demonstration of biased random walk.
Each star is a student performing a biased random walk towards the apple pie. The biased random walk is shown for only three students (dark black stars). Each student performed four runs (arrows) and four tumbles, with the black circle tumble being the end location of the student. The student will always tumble after a run but if they are facing apple pie, they run for ten steps; if facing away from apple pie, run for two steps; or if facing in between, run for five steps. More of the students have migrated towards the apple pie in the biased random walk than in the random walk. This is evident by two of the three students (black circle end tumbles) reaching past the three-quarter line towards the apple pie.
Students: To get your slice of the pie, you asE.coli now can perform chemotaxis and can direct your movements towards the apple pie by doing whatE.coli does, a biased random walk (2). You still have to both run and tumble but now you can alter the length of the runs. If you are facing the apple pie you can smell it (sense attractants) and you will run longer (10 steps). You still have to tumble. If you find yourself facing in an opposite direction from the apple pie, then you will run but for a shorter period of time (2 steps). If you are facing perpendicular to the apple pie then you will run for 5 steps. No matter what direction you are facing you have to tumble after your run. Basically, the lengths of your runs are altered depending on your direction in relation to the apple pie (Fig. 2).
Instructors: Let the students do this for a couple of minutes. Stop the students and ask them to check where they are. Are they closer to the apple pie now that they can “sense” it? How many students compared to the last activity are near the apple pie? Many more students in the biased random walk should be near the apple pie than in the regular random walk. After the activity, the class learns that in the chemotaxis system, E.coli uses protein receptors to sense attractants or repellents and a phosphorylation protein signaling cascade to direct flagellar movements (4,5).
CONCLUSION
In the classroom, I consistently refresh the students’ memory about this activity as we discuss how the proteins CheA and CheY regulate chemotaxis and direct flagellar movement (5). We discuss how CheY binds to the flagellar motor to direct movement in a counterclockwise (CCW) or clockwise (CW) fashion (5). I believe this short activity greatly aids in their understanding of the biased random walk in the context of chemotaxis and helps them understand the more detailed concepts. This is evident on their answers on exam questions.
I include questions that pertain to chemotaxis material on lecture exams. For example, Question #15 shown below was answered correctly by 27 out of 30 students (90%) on a lecture exam. The overall mean on this exam was 66%.
- 15) A bacterium is mutated and canonly rotate its flagellum in the CW (clockwise) direction. Therefore this organism can move in this manner:
- runs (swims) only
- tumbles only
- less tumbles, more runs
- more tumbles, less runs
- runs and tumbles with no chemotaxis
Students were asked the following question at the end of semester course evaluations:What aspect(s) of your classroom experience (course, professor, etc.) helped your learning most? Student responses included re-explanation of text book matter with drawings, animations and demonstrations. One student commented that “the current events helped and the way she tried to teach us the same material in a few different ways, visually, auditory and by having us write the information out was great!”
REFERENCES
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