Summer Science Academy Experiment:

Disinfection

 

GRADES: 6-12

 

TIME NEEDED: ½ hour (day 1); ½ hour (day 2)

 

Proper handwashing performed by employees of hospitals or medical centers is the most effective method of controlling infections. This first line of defense has become even more important due to the many antibiotic resistant organisms that inhabit most hospitals.

 

Simple hand washing, using bath soap and water, is not effective against many microorganisms. A layer of oil as well as the structure of the skin prevent the removal of microorganisms by simple handwashing. Using various soaps and disinfecting agents, you will explore the effectiveness of handwashing to remove microorganisms from your skin. You will also determine the effectiveness of disinfectants in removing microorganisms from surfaces.

 

Materials Needed

 

Soaps: Phisoderm, anti-bacterial bath soap, Ivory, lab soap (Exedine)

Disinfectants: 70% alcohol, 90% alcohol, 10% Clorox, Pine Sol

Tryptic Soy Agar plates

Sterile cotton swabs

 

NOTE: This experiment is best done with several groups of students, so that you can compare results.

 

Experimental Procedure

 

A. Hand Disinfection

 

1. Take an agar plate and mark the bottom into 2 sections.

2. Place two fingers on one section of the plate.

3. Each lab group will choose a different agent to wash their hands with. One person will wash hands for 10 seconds, and the other person will wash hands for one minute.

4. Being careful not to re-contaminate your hands, place the same two fingers on the other section of the agar plate.

5. Incubate the plates at 37C overnight.

6. Compare the growth on the agar plate of washed and unwashed fingers. Record your results in the table and determine the most effective agent and time of washing for disinfection.

 

 

PLATE GROWTH AFTER WASHING

10 sec. 1 min.

 

Type of Soap

 

7. Pick several colonies from the "after washing" section and streak them onto a new plate. Incubate the plate overnight at 37C. Do a Gram stain on several of the isolated colonies and try to characterize the bacteria that remained on your hands following washing.

 

B. Surface Disinfection

 

Determine the effectiveness of several antibacterial agents to disinfect the lab bench top.

1. Take an agar plate and mark the bottom into 2 sections.

2. Use a sterile swab to wipe a section of your lab bench top. Rub the swab on one section of the agar surface.

3. Choose one of the disinfectant agents to wipe off the bench top.

4. Use a new sterile swab to take a sample of the disinfected bench top. Rub the swab on the remaining section of the agar plate.

5. Incubate the plate at 37 C overnight.

6. Compare the growth on the agar plate on the two sections. How effective was your disinfectant in killing microorganisms?

7. Pick several colonies from the "after washing" section and streak them onto a new plate. Incubate the plate overnight at 37C. Do a Gram stain on several of the isolated colonies and try to characterize the bacteria that remained on the bench top following washing.