4 Treatment and Prevention

Activity 1

Protection from Infection

Introduction

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.

Materials

  • 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

Advance preparation

Steps 1 and 2 can be done several days in advance. Contaminate the candy (step 4) just prior to use.

  1. 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.
  2. Transfer about 5 mL of sterile nutrient broth to each tube either by pipetting or by careful pouring.
  3. Prepare yeast solution by dissolving 1 package of yeast in 100 mL sterile warm water (boiled water cooled to 30°C).
  4.  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.
  5. 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.
  6. 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)

  1. 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.
  2. 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.
  3. 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)

  1. 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.
  2. Count off within your group and write your number on an agar plate using a wax pencil.
  3. Wash your hands thoroughly.
  4. 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.
  5. 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.

  1. 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).

petri dish
  1. Place the swab or loop in the area designated for disposal or clean-up.
  2. Cover the agar plate, turn it upside down (agar side up), and store it at room temperature (approximately 25°C) to incubate for 24 hours.
  3. Wash your hands thoroughly, then rinse with a dilute solution of disinfectant, if available.
  4. Predict the outcome of the experiment for each member of your group and be prepared to explain your reasoning for the prediction. Will the agar plate swabbed by student 6 in group A have more, less or the same amount of growth as student 1? Will the agar plate swabbed by student 6 in group B have more, less or the same amount of growth as student 1?What will the agar plate of the final student in your group who did not shake hands look like? What purpose does the agar plate of the final student serve?
  5. After 24 hours of incubation, examine your plate and those of others in your group. Record whether or not there is growth on each of the plates and if so, how much (you can use a scale of +++, ++, +, or -). Be prepared to compare them to your predictions and explain your findings.
  6. Activity Extension – This activity can be used to test the effectiveness of antibacterial soaps (as compared to regular soap) and the effectiveness of hand washing procedures such as time of washing or scrubbing with a brush vs. just washing without scrubbing. To carry out these variations you will need more groups, one group for each variable.

Discussion

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:

  1. Which plate or plates have the most growth? The least? Would student 1 be sicker than student 5? Why or why not?
  2. 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?
  3. 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?
  4. What methods of protection from a contagious disease does this activity demonstrate?
  5. What variables related to hand washing could you investigate with this experimental approach (e.g. time of washing, scrubbing techniques, different types of soaps)?

Infectious diseases can be transmitted in a number of ways; through direct contact with an infected individual, by inhalation of droplets emitted by a cough or sneeze from an infected individual (aerosol), through ingestion of contaminated water or food, or from bites by vectors such as mosquitoes and ticks that carry infectious agents.

Direct contact is a very inefficient method of transmission for infectious disease (such as Ebola) because it requires an infected individual to come in physical contact with an uninfected individual. Diseases spread by airborne droplets from an infected person (such as measles) are much more contagious. (Learn more about modes of viral transmission and infectivity of different infectious diseases in Module 3 – Viruses Go “Viral”)

Diseases spread by direct contact may also be the simplest to avoid, although changing human behavior and certain traditions such as burial rituals is not often easy. In this activity you model an infectious disease spread by direct contact and determine ways to avoid being infected.