Every time you touch something you are probably picking up new bacteria and leaving some behind. This is how many infectious diseases spread—we share our bacteria with everyone around us! Even bacteria that lives safely on our skin can make us sick if it gets inside our bodies through our mouths or cuts and scrapes. This is one reason why it is so important that we wash our hands frequently and well.
What kind of soap works best for cutting down on the bacteria on our hands? You can test this by growing some bacteria cultures using agar and petri dishes. (Scroll down for ideas of other things you can test, too!) Adult supervision is required when working with bacteria.
While most environmental bacteria are not harmful to healthy individuals, once concentrated in colonies, they can be hazardous. To minimize risk, wear disposable gloves while handling bacteria, and thoroughly wash your hands before and after. Never eat or drink during bacteria studies, nor inhale or ingest growing cultures. Work in a draft-free room and reduce airflow as much as possible. Keep petri dishes with cultured mediums closed—preferably taped shut—unless sampling or disinfecting. Even then, remove the petri dish only enough to insert your implement or cover medium with bleach or 70% isopropyl alcohol. When finished experimenting, seal dishes in a plastic bag and dispose. Cover accidental breaks or spills with bleach or alcohol for 10 minutes, then carefully sweep up, seal in a plastic bag, and discard.
What You Need:
What You Do:
- Prepare the agar according to the directions on the label, then pour enough to cover the bottom of each petri dish. Cover the dishes and let them stand for about an hour until the agar has solidified again. (If you aren't going to use them right away after they have cooled, store them upside down in the refrigerator.)
- When your petri dishes are ready, collect some bacteria from your hand or the hand of a volunteer. (Make sure the person hasn't washed his or her hands too recently!) Do this by rubbing the sterile swab over the palm in a zigzag pattern.
- Remove the cover from the petri dish and lightly rub the swab back and forth in a zigzag pattern on the agar. Turn the dish a quarter turn and zigzag again. Cover the dish and repeat steps two and three for the other dish, using a new sterile swab. Label the dishes 'Test' and 'Control.' (You may want to do more than one test dish, so you can compare the results.)
- Cut the blotter paper into small 'sensitivity squares.' Use permanent ink to label the squares for the different types of hand cleaners you are going to test, e.g., 'R' for regular soap, 'A' for antibacterial soap, and 'S' for hand sanitizer. Using tweezers, dip each square into the appropriate cleaner. Blot the excess cleaner on a paper towel and then place the squares on the agar in the 'Test' dish. (Spread the squares out so there is distance between them.) Add one square of plain blotter paper to test if blotter paper by itself has any effect. Don't put any squares in the 'Control' dish - this one will show you what the bacterial growth will look like without any soap.
- Put the dishes in a dark, room-temperature place like a closet and leave them undisturbed for a few days.
After 3-7 days, take your petri dishes out and observe the bacteria growth (without removing the lids). There might be some mold growth, too, since you may have swabbed microscopic mold and fungi spores along with the bacteria. Compare the amount of bacteria in the control dish to the amount in the test dish. Next, compare the amount of bacteria growth around each paper square. Which one has bacteria growing closest to it? Which one has the least amount of bacteria growing near it? If you did more than one test dish, are the results similar in all the test dishes? If not, what variables do you think might have caused the results to be different?
(Remember to use care when experimenting with bacteria. The kind of bacteria you're using in these projects are types that are normally present in your house, but you are culturing them in greater numbers than usual, and this can be hazardous. When you're finished with your experiment, pour a little bleach into each petri dish, seal the dishes in a plastic bag, and throw them away.)
Which soap inhibited bacteria growth the most? Which one would you recommend using when you wash your hands? What other substances could you test for antibacterial effects? Try some of these ideas:
- Household cleaners. Which household cleaners work best against bacteria? Try swabbing a surface in your home, like the kitchen sink or a toilet, and then use sensitivity squares to test different cleaners such as Lysol, bleach, Windex, etc.
- Natural substances. Test to see if garlic really has antibacterial properties. What about tea tree oil, or red pepper, or curry?
- Mouthwash. Swab your teeth and gums and see how well toothpaste or mouthwash work against the plaque-causing bacteria on your teeth.
- Antibiotics. Use an antibiotic disc set to see what different antibiotics can do against bacteria. For a more advanced project, learn how gram staining relates to the use of antibiotics.
Don't stop there—there are lots of other projects you can do with bacteria
- Have you heard people say that dogs' mouths are cleaner than humans'? Design an experiment to test whether this is really true!
- Some band-aids are advertised as being antibacterial. Test to see if they really work better than regular band-aids at inhibiting bacteria.
- Is it safe to keep refilling a water bottle without washing it? Test a sample of water from the bottom of a water bottle that has been used for a couple days and compare it to a sample from a freshly-opened, clean water bottle. You can also test to see if a bottle gets more bacteria in it if you drink with your mouth or with a straw.
- Does bacteria grow in your shoes? Is there a difference in bacteria growth between fabric shoes and leather? Do foot powders work to cut down on bacteria?
- Does bacteria grow on your toothbrush? What are some ways you could try to keep it clean? Mouthwash? Hot water?
- Some people recommend getting a new mascara every 6 weeks because bacteria can grow in the tubes. Test this by comparing bacteria growth from old mascara and new, unused mascara.
- What about good bacteria? Try making some homemade yogurt with this kind!
As you experiment, you'll notice that the bacteria growth in your petri dishes will often be different colors or textures. These are colonies of different types of bacteria. If you have a microscope, look at a sample of each colony and observe the differences. Bacteria come in three different shapes; see if you can distinguish these shapes under the microscope.