Have students read the Introduction to the module in Activity 1. With polio as an example, students are introduced to treatment and prevention of viral diseases. Polio is an excellent example of a contagious disease that has been greatly reduced worldwide as a result of vaccination programs. Students consider the reasons why polio could be eradicated worldwide but how certain barriers prevent this.
In this activity students model the spread of an infectious disease and investigate how the simple act of handwashing can prevent an infection. The activity involves the contamination of an object—a hard candy—with a microorganism (in this case, a benign fungus—baker’s yeast). Students spread the “disease” by passing the candy from hand to hand. Culturing the organisms from their hands identify “Infected” individuals. Step 12 of the Procedure provides options for extending the activity.
- Nutrient agar plates (1 for each student)
- 1 package baker’s yeast
- Hard candy
- Household chlorine bleach
- Sterile or distilled water
- 100 mL sterile nutrient broth
- 1 small test tube and tongs/tweezers (for each group)
- 1 large beaker (1000-mL)
- sterilized tweezers or tongs
- Soap and warm water
For each student:
- 1 pair of safety goggles
- 1 sterile cotton swab or inoculating loop
- 1 nutrient agar plate
For each group of eight students:
- 1 piece of contaminated candy (hard candy)
- 1 wax marking pencil (grease pencil)
- 1 test tube containing nutrient broth
For the class:
- Soap and warm water or hand sanitizer solution
- yeast solution
- sterilized tweezers or tongs
Steps 1 and 2 can be done several days in advance. Contaminate the candy (step 4) just prior to use.
- Sterilize test tubes (one test tube for every group of eight students) and tweezers or tongs by boiling in water for 10 minutes You may also use disposable plastic test tubes.
- Transfer about 5 mL of sterile nutrient broth to each tube either by pipetting or by careful pouring.
- Prepare yeast solution by dissolving 1 package of yeast in 100 mL sterile warm water (boiled water cooled to 30°C).
- Contaminate one piece of candy for each group of 8 students by dropping it into the yeast solution and swirling the mix. Remove with sterile tongs or tweezers.
- Designate a container or bag (for the class or for each group) into which all the contaminated candies and swabs will be placed after use.
- Decide on the best means for all students to wash and/or disinfect their hands before they start the Procedure. If you don’t have easy access to running water, you may want to prepare stations with towels and either bowls or buckets filled with soap and warm water. You can also use a hand sanitizing solution.
Procedure (for the teacher)
- Divide the class into two or more large groups so that each group has at least eight students. Make sure that each group has a set of the materials required. Have them read the Procedure before beginning.
This experiment works well but is sensitive to bacterial contamination. Good sterile techniques will keep contamination to a minimum. Strategies that may help include the following:
- Minimize the time that the agar plate is open to the air during swabbing; be sure to keep fingers out of the plate;
- Incubate plates at room temperature (no higher than 25°C) for 24 hours; higher temperatures or longer incubations may allow for bacterial contaminants to grow;
- Encourage students to wash their hands well since hands are the breeding ground for a great variety of contaminants.
- After students have observed their results have them record their observations, discuss their findings with their group, and prepare for a class discussion.
Procedure (for the students)
- Your teacher will divide your class into at least two groups of eight or more. One group is designated Group A, the other Group B.
- Count off within your group and write your number on an agar plate using a wax pencil.
- Wash your hands thoroughly.
- If your number is 1, the instructor will give you a piece of candy soaked in the “contaminant,” a yeast solution (a benign fungus). Roll it around in your right hand until your palm and fingers are very sticky. Put the candy in the disposal area designated by the instructor.
Group A - Shake hands with student number 2 in your group. Student 2 shakes hands with student 3, and so on, until all but the last person in the group has had a handshake. After shaking hands avoid touching or rubbing hand on pants or shirt.
Group B - Shake hands with student number 2 in your group. Student 2 shakes hands with student 3. Continue shaking hands through student 5. Student 5 should then wash his/her hands thoroughly in soap and water, scrubbing well. Student 5 then shakes hands with student 6, student 6 shakes hands with student 7, and so on, until all but the last person in the group has had a handshake. Downloadable diagrams of this step are provided below.
Take a sterile cotton swab or inoculating loop, dip the cotton or loop end in the nutrient broth in the test tube, then swab your right hand with it. Carefully open your agar plate and gently rub the loop or streak the swab across the surface of the agar. Roll the tip as you streak to transfer as much of the material gathered from your hand as possible to the agar. Swab in a zigzag pattern (see diagram).
The intent of this discussion is to have students consider how diseases spread by direct contact can be prevented. Students should recognize two methods of prevention, one by having no direct contact with an infected individual (final student in each group) and another by washing with soap and water (student 5, group B). Students should also recognize that the final student in each group is an experimental control. Because the student had no direct contact with an infected individual his/her agar plate should show no growth (-). If growth appears, then the materials (e.g. the nutrient agar plate or the swabs) were contaminated most likely by bacteria.
Students should understand that though the growth may be less on plates from individuals at the end of the handshaking sequence, any growth at all means that the individual is infected. Students may think that less growth would mean a less “infected” individual and therefore a “less sick” individual. To address this possible misconception you may wish to provide an example. If one student has a cold and passes it to a second student who then infects a third student, is there any way to predict how sick each student might be relative to the others? Many factors such as the health of the person before the infection, how well-nourished he/she is, and the responsiveness of the immune system are involved in how ill an individual will become with an infection. Being fourth, fifth, or one hundredth from the original source of the infection is generally not a factor.
Have students present their findings and explanation of the results.
You may want to initiate discussion with questions such as:
- Which plate or plates have the most growth? The least? Would student 1 be sicker than student 5? Why or why not?
- Did the results agree with your prediction? If not, why do you think they differed? Were any of the results surprising? If so, why were they surprising?
- Why was the last person in your group instructed not to shake hands with anyone? How is that person’s plate different from the others? What does this result tell you?
- What methods of protection from a contagious disease does this activity demonstrate?
- What variables related to hand washing could you investigate with this experimental approach (e.g. time of washing, scrubbing techniques, different types of soaps)?