Summer Research Program for Science Teachers

By Wendy Rokach

Determining the Effectiveness of Handwashing

Objectives: Students will grow bacterial colonies and determine the effectiveness of handwashing. Students will grow bacterial colonies and determine the effectiveness of antibiotics in killing the bacteria.

Background Information:

Most bacteria can proliferate rapidly through binary fission, one type of asexual reproduction. [9-12 Content Standard C- The Cell] Under optimal conditions, certain bacteria can divide as rapidly as once every 20 minutes. This ability to reproduce at such great speed has proved invaluable to geneticists. Initially, scientists were not sure whether bacteria would be useful subjects for genetics research because it seemed that their method of reproduction was too simple. Research found that although bacteria do not reproduce sexually, like plants and animals, they do have the mechanisms for transferring genes from one individual organism to another. These mechanisms allow bacterial strains to become resistant to antibiotics. [9-12 Content Standard C- Biological evolution] [9-12 Content Standard F- Natural and human induced hazards]

While all bacteria are not disease causing bacteria, many disease causing bacteria respond to treatment by antibiotics, which are used to kill or slow the growth of bacteria. In this activity, students will study the effectiveness of some over the counter antibiotics. Students will also investigate some of the microorganisms that are normally present on the human skin. These bacteria are not pathogens. The students will compare bacterial growth on washed and unwashed hands.


Groups of 4

2 agar filled petri dishes

marking pencil



Distilled water

Bar soap, antibacterial soft soap

Antibiotics [Teaching Standard D- Make accessible science materials]


1. Prepare nutrient agar and pour into sterile, disposable plastic petri dishes. Allow time to set.

2. Prepare antibiotics in properly clearly labeled bottles.

3. Have students divide two nutrient agar plates into four quadrants.

4. Label the quadrants of each plate 1 through 4 using a wax marking pencil.

5. Label one plate "handwashing" and the other "antibiotics".


2 students will follow handwashing procedures and 2 students will follow antibiotics procedure.

Upon completion of experiment each group of four will discuss their separate procedures and results and will prepare a written report incorporating both parts of the experiment. [Teaching Standard E- Nurture collaboration among students]

Part I: Handwashing

1. Student A touches quadrant 1 and student B touches quadrant 2 with fingers.

2. Student A washes hands with bar soap and touches quad. 3 with finger and student B washes with anti-bacterial soft soap and touches quad. 4 with finger.

3. Incubate the plates inverted at 35C or room temp. until the next period. (about 24-48 hrs. for growth)

Part II: Antibiotics

1. Have each student lightly moisten a Q-tip with distilled water. Each student should rub the cotton tip lightly across a surface in the classroom, such as a desk, doorknob, or pen cap. Have students roll the cotton swab as they move it over the surface to collect the maximum number of bacteria.

2. Each student now streaks the cotton swab over 2 quadrants of the petri dish.

Student A streaks quad 1 and 2; student B streak quads 3 and 4.

3. With a clean Q-tip have student A moisten the tip with antibiotic 1 and student B moisten the tip with antibiotic 2. Have each student gently streak the tip on one quadrant. Student A streaks quad 3 and student B streaks quad 4.

4. Have each student initialize their 2 quadrants and which had antibiotics and which does not.

Enforce proper labeling!!!

5. Incubate the plates inverted at 35C or room temp until next period.

Discussion Questions:

[Teaching Standard B- Orchestrate scientific discourse]

1. Why is it necessary to sterilize the petri dishes before we start this investigation?

2. The microorganisms that are normally found on human skin are not pathogens. Why does a surgeon scrub for 2 to 5 minutes with an antiseptic soap before operating?

3. Which antibiotic appears to be the most effective? How were you able to tell? [9-12 Content Standard A- Formulate explanations using evidence]

4. How were you able to tell if more than one type of bacteria was growing on the agar in the petri dish?

[Content Standard Unifying Concepts- Change, constancy, and measurement]

5. Are there any diseases that you know of that can be transmitted by hands?

6. Make a sign for the school bathroom that will get your fellow students to wash their hands. Why is it important to always wash your hands when leaving the bathroom?


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