Most of the microbes on our bodies and other surfaces are harmless, but some are pathogenic or disease-causing. For this reason, we want to control the number of microbes around us. The odds of becoming infected increase with the number of microbes on surrounding objects. But what can we do to affect the number of microbes on surfaces around us?
In this activity, you will test a chosen fomite for the presence of microbes and the effects of a disinfectant by growing colonies of bacteria in a medium on petri plates. A medium has food, vitamins and salts that help microbes grow. You usually don’t see bacterial colonies like those that form on petri plates on everyday surfaces. That’s because there is rarely such a perfect concentration of nutrients on fomites in nature.
Note: This is an activity that you will start on one day and finish on a different day.
What To Do:
1. If you have long hair, tie it back to keep it from dangling into the petri plates as you’re working. Wash your hands. Clean your work area by dabbing, not pouring, disinfectant solution onto a paper towel and swabbing your area. Set out your petri plates but DO NOT OPEN THE PLATES UNTIL YOU'RE TOLD.
2. Choose an object in the room (doorknob, picture frame, toy, kitchen counter, TV remote control, coin, etc.). Take one unopened petri plate and using your grease pencil or marker, divide the bottom of the plate into four equal sections. Write the object’s name across the top and label the sections 1 through 4. Open the box of cotton swabs and select one being careful not to touch the tip. Swab your chosen object with all sides of the swab tip by turning and twisting the swab as you move it across the object’s surface.
3. Now open the lid of the plate and GENTLY make four streaks on the plate’s surface as shown in the illustration, starting in the section labeled "1" and continuing streaking in order of the sections, making your last streak in section
4. Use firm, but GENTLE pressure and do not retrace your previous streaks. Your streaks should make only very slight impressions in the agar—don’t gouge. Close the plate and seal it shut with two pieces of tape placed along the side—don’t cover over the top with tape or you won’t be able to see the inside of it well.
5. Divide your third petri plate into 4 numbered sections and label it with the name of the disinfectant you’ve chosen (e.g. "Bleach"). Use your chosen disinfectant to clean the other half of the object you swabbed. Using another new cotton swab, swab the area for microbes. Repeat the process of streaking the plate. Close and seal the plate.
6. Soak the used cotton swabs in disinfectant and throw them away. Place your plates in an out of the way spot and let them incubate at room temperature for two days. Clean your work area with disinfectant solution. Wash your hands.
7. After two days have passed, look at your initial petri plate. Do not open it. Examine your other petri plates in turn without opening them. Create a table that compares the plates made before and after cleaning the object (see sample table below). Be sure to indicate whether microbes grew in each streak.
8. Note: An adult supervisor preferably should do this step. Very carefully open the petri plates in a sink and flood them with undiluted bleach or alcohol. Let stand for an hour and then rinse them out thoroughly, tie them in a plastic bag and throw them away. Be sure not to touch the plate surfaces when you open them and wash your hands thoroughly after handling the plates. Clean your work area with disinfectant solution.
Note: You can try variations of this activity to test different things. For example, you may want to test the sterilizing power of different types of soaps or cleansers. To do this, you would pick a large object, such as a kitchen counter, and divide it into 4 or more sections. Swab one section that you don’t clean. Wipe each of the other sections with a different cleanser and swab each, using a different petri plate for each section. Compare the plates after a few days.
This experiment is based on an activity developed by the National Association of Biology Teachers.