How Well Do Disinfectants Work?
| Difficulty | |
| Time Required | Short (2-5 days) |
| Prerequisites | None |
| Material Availability | Specialty items |
| Cost | Very Low (under $20) |
| Safety | Must follow general safety precautions for handling microorganisms |
Abstract
Germs are everywhere! We try and clean our home with disinfectants, but do we really get rid of all the germs? Find out with this "hands-on" experiment.Objective
In this experiment you will test several disinfectants to see which ones work best to decontaminate the surface of a cutting board.
Credits
Sara Agee, Ph.D., Science Buddies
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Last edit date: 2013-01-10
Introduction
When your mom tells you to wash your hands before dinner, she is thinking about all the germs you might have on your hands from touching the world around you. Germs are everywhere! Germs, or microorganisms, that cause illness and disease can grow on many surfaces called fomites. Here is a good description of fomites and how they harbor germs from the American Society for Microbiology:
"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." (ASM, 2001)
 |
| An agar plate showing colonies of microorganisms that have been isolated from a deep-water sponge. (Wikipedia contributors, 2006; original source of image from NOAA Ocean Explorer) |
How well do disinfectants work to clean surface bacteria? In this experiment you will use a cutting board as a fomite and test the use of different brands of disinfectants. By culturing bacteria from the surface after you have cleaned it, you will find out how well the disinfectant worked. Which disinfectants will work the best?
Terms and Concepts
To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!
- germs
- microorganisms
- bacteria
- fomite
- disinfectant
- How much bacteria can contaminate the surface of a fomite?
- Will the disinfectant work to kill the bacteria?
- Will some disinfectants work better than others?
Bibliography
- ASM, 2001. "Fun With Fomites," American Society for Microbiology [accessed July 3, 2006]
http://www.microbe.org/experiment/fomites.asp - ASM, 1999. "Stalking the Mysterious Microbe," American Society for Microbiology [accessed July 3, 2006]
http://www.microbe.org/index.html - Wikipedia contributors, 2006. "Bacteria," Wikipedia, The Free Encyclopedia [accessed July 3, 2006] http://en.wikipedia.org/w/index.php?title=Bacteria&oldid=66380071
- Wikipedia contributors, 2006. "Disinfection," Wikipedia, The Free Encyclopedia [accessed July 3, 2006] http://en.wikipedia.org/w/index.php?title=Disinfection&oldid=64561480
Materials and Equipment
- large plastic cutting board
- permanent marker
- slice of lunch meat (ham, turkey, bologna)
- latex gloves
- unopened (sterile) cotton balls
- tweezers
- unopened (sterile) Q-tips
- petri dishes pre-filled with nutrient agar (available from Science Kit & Boreal Laboratories, $10.50 for 10 pre-poured plates of nutrient agar, catalog #WW6564600 can be ordered online at www.sciencekit.com or by phone at 1-800-828-7777)
- small Dixie cups for holding disinfectant solutions
- 5 different disinfectant solutions (some examples below) and water as a control:
- sterile, bottled water
- 10% bleach (mix 1 part bleach with 9 parts water)
- 70% alcohol (mix 7 parts rubbing alcohol with 3 parts water)
- Lysol
- antibacterial soap
- Pine-Sol
- (optional) a digital camera
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Experimental Procedure
- Using the permanent marker, divide the cutting board into six sections, numbering each section #1–6 like this:
- Put on your gloves and wipe the piece of lunch meat all over the surface of the cutting board. Wipe evenly over the entire surface in circular motions. Leave out overnight, gross!
- The next day you will cleanse each section of the cutting board with a different disinfectant and then culture the bacteria from each section on a nutrient agar petri dish.
- Prepare your disinfectant solutions by numbering six small cups #1–6 using your permanent marker. Each numbered cup will match one section of your cutting board fomite.
- Fill each cup with a different disinfectant solution and write it in a data table. You should fill the first cup with water as a negative control:
Number Type of Disinfectant Number of Colonies Other Observations 1 water (control) 2 3 4 5 6 - Using the tweezers, grab a cotton ball from a NEW, unopened bag of cotton balls. Dip it into one of the solutions, and rub it on the surface of the cutting board in the matching numbered section. Be careful not to let the solution run into another section!
- After each application, throw the cotton ball into the trash and dip your tweezers into an extra cup filled with full strength rubbing alcohol.
- Repeat steps 6 and 7 until you have applied a different disinfectant to each square of the cutting board. Allow the board to dry completely.
- When all of the sections of the cutting board are dry, you are ready to culture bacteria from each disinfectant treatment. Prepare your agar plates by numbering the lids #1–6 to match the sections of your cutting board. Arrange the plates on a cookie sheet lined with clean paper towels. DO NOT open the lids yet or you will contaminate your cultures!
- Using the tweezers, grab a clean Q-tip from a NEW, unopened box of Q-tips. Holding one end only, swipe the other end across one section of the cutting board using a circular motion. DO NOT allow the tip of the swab to contact anything else!
- With your free hand, open the lid of the matching agar petri dish and swipe the Q-tip gently across the agar surface using a zig-zag motion. Immediately replace the lid of the petri dish and secure with a few pieces of clear tape. DO NOT set the lid down while you are streaking the agar because this can contaminate the lid and change your results!
- Repeat steps 10 and 11 until you have swiped each section of the cutting board onto a separate agar plate.
- Leave the plates on the cookie sheet in a warm place for 1–2 days, until bacterial colonies are visible.
- Count the number of colonies on each plate and write your results in the data table, along with any other observations you have.
- Draw a picture of each plate, noting the size and color of the colonies. If you have a digital camera, you can also take a picture of each plate for your poster.
- Graph your results and compare the activities of the different disinfectants. Which sections of the cutting board had the most colonies? Which had the least? Which disinfectants worked the best? The worst?
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Variations
- Disinfectants work to discourage bacterial growth by killing bacteria or retarding growth processes. There are other ways to affect bacterial growth too, like putting your food in the refrigerator or cooking food to a high temperature. Try testing the growth of bacteria in different temperatures. Will they grow in your refrigerator?
- Bacterial colonies come in all shapes and sizes. This is because there are many different species of bacteria with their own characteristics. A single bacteria may be too small too see without a high powered microscope, but you can see a colony with your naked eye. Take some swipes from around your home too see how many different types of bacteria you can find. Can you use clues found by observing the colonies to tell them apart?
- When we think of bacteria we think about germs. Where do you think the most bacteria in your home are? Try swiping different areas of your home to test how much bacterial contamination is there. Test door handles, toilet seats, and trash can lids. Which places in your house have the most germs? For more information about germs, see the Science Buddies experiment Germ Invasion.
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