Epidemic! The Natural History of Disease.  At the San Diego Natural History Museum

Experiment: Fun with Fomites

For intermediate and advanced students; time: 2-3 days. Do day 1 before the museum visit and follow-up activities on day 3 after the museum visit

Fomites? What are fomites? This is a term for any inanimate object that can carry disease-causing organisms. Your cutting board, kitchen sink, the change in your pocket, and even that pen you keep putting in your mouth are all fomites.

Very few things we encounter in our everyday activities are sterile, or microbe-free, including us. At birth, microbes immediately begin colonizing our bodies as they do most every object in the world. They float around until they come in contact with a surface that offers food and shelter. You are most likely to find microbes in and on dark, moist objects that frequently come into contact with food, dirt, or vegetation. Bathroom surfaces, hairbrushes, refrigerators, kitchen sinks, and cutting boards often have lots of microbes on them. But doorknobs and walls have fewer because they are nutrient-poor and dry.

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.

What You'll Need
  At least 3 sterile petri plates prepared with nutrient agar (can be ordered from Carolina Biological Supply Co. by calling 800/334-5551) Note: you will need 3 plates for each fomite you test. If you wish to test more than one object for microbes, order 3 more plates for each additional fomite.
  Unopened box of sterile cotton swabs
  Paper towels
  Cellophane tape
  Permanent marker or grease pencil
  A disinfectant such as 70% alcohol solution (mix 7 parts alcohol to 3 parts water), 10% bleach solution (mix 1 part bleach to 9 parts water), liquid soap, Lysol® or other household cleanser

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 agarčdon'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 easily.
4. Divide a second unopened petri plate into 4 sections numbered 1 through 4 and label it "Control." Clean half of the object you swabbed with a paper towel dampened with plain waterčjust wipe a couple of times; don't scrub. Using a new cotton swab, swab the cleaned area for microbes. Open the lid of the second plate and GENTLY make 4 streaks on the plate's surface, following the order of the numbered sections as you did previously. Close the plate and seal it.
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. 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.

Questions for Discussion

1. Which plate grew the most and biggest colonies? Why do you think that is?
Unless the object you tested had been cleaned shortly before you swabbed it, you most likely grew some nice colonies of bacteria in your first plate. Your second plate, the one you swabbed after wiping the object with water, may have also grown some colonies, though there may have been less growth than on the first plate. Your third plate, the one you swabbed after you cleaned the object with disinfectant, likely grew few colonies if any.

2. Do you see a pattern in the size and amount of colonies in each plate?
You probably got the most colony growth in section 1 of each plate that grew any bacteria. The number of colonies in each of the four sections of the plates likely decreased from section 1 to 4. This is because each time you swabbed a section of the plate, there were fewer bacterial cells remaining on your cotton swab. So by the time you swabbed section 4, there were only a few cells left to get onto the plate and grow into colonies. Why do microbiologists swab petri plates this way? Because they want to grow individual colonies separated by space from other nearby colonies. By examining the shape and color of these individual colonies, they can tell certain things about the bacteria, such as what type they are, whether they're normal or mutated, etc.

3. How can we control microbial contamination? Did your petri plate grow bacterial colonies after you wiped the object with just water? Probably. This shows that while plain water can help get rid of some microbes, it doesn't necessarily get rid of all of them. But you didn't grow too many microbes in the dish you swabbed after wiping the object down with your chosen disinfectant. Disinfectants are chemicals that kill microbes or at least lift them up off a surface so they can then be washed or wiped away. Disinfectants are one of the ways we can control microbial growth. Heat and cold are other means--you can sterilize water by boiling it, and you can keep food from going bad by refrigerating or freezing it. Irradiation and antibiotics are other control methods.

4. If you tested more than one fomite, which one grew more microbes? Why is that?
Depending on what objects you tested, you may have seen a difference in the amount of bacterial growth. Objects that are kept in moist and/or dark places or that come into frequent contact with food, dirt, vegetation, or bodies of living creatures often contain more microbes than other objects such as walls, ceiling light fixtures, door frames, etc. that are generally dry.


What did you find out by doing the activity?

Before doing "Fun with Fomites," did you know

  • How people get diseases?
  • What microbes need to survive on surfaces?
  • How microbes go from one object to another?
  • Any products that can be used to reduce or eliminate harmful microbes from surfaces?

From this activity, did you discover

  • How microbes can multiply?
  • How diseases can be caused by harmful microbes on surfaces?
  • How effective products are at disinfecting?
  • What happens if harmful microbes become resistant to disinfectants?
  • Some objects in your house that could be choice surfaces for harmful microbes?
  • How to reduce the number of harmful microbes?

Source: The National Association of Biology Teachers for MicrobeWorld Activities, a community outreach initiative of the Microbial Literacy Collaborative. See www.microbeworld.org.

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from the American Museum of Natural History Epidemic! exhibition

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