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Agar & Petri Dishes
Bacteria BasicsThis experiment will get you started with the basics of studying bacteria by growing a bacteria culture using a sample from the cheek cells inside your mouth. Remember to treat every bacterial culture with great caution! Adult supervision recommended.
Make a culture dish using the following directions: Prepare agar according to the directions on the label, then pour enough into a petri dish to just cover the bottom. Rotate the dish to obtain even surface coverage. Cover it immediately and let stand until firm. Store upside down in the refrigerator until ready to use.
Once the culture dish is prepared, use a sterile cotton swab or inoculating needle and swab the inside of your cheek. Very gently rub the swab over the agar in a few zigzag strokes and replace the lid on the dish. You'll need to let the dish sit in a warm area for 3-7 days before bacteria growth appears. Record the growth each day with a drawing and a written description. The individual bacteria are too tiny to see without a high-power microscope, but you can see bacteria colonies. Distinguish between different types of bacteria by the color and shape of the colonies.
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.
You need two culture dishes for this experiment, in which you'll demonstrate how antibacterial agents (such as antibiotics and household cleaners) affect bacteria growth. Leave the dishes with their lids off in a room-temperature location. Leave the culture dishes exposed for about an hour. While you wait, cut small squares of paper (blotter paper works well), label them with the names of the antibacterials you're going to test (e.g. "L" for Lysol, "A" for alcohol, etc.),, and soak each in a different household chemical that you wish to test for antibacterial properties. If you have time, you might also experiment with natural antibacterial agents, such as tea tree oil or red pepper. Wipe off any excess liquid and use tweezers to set each of the squares on a different spot in one of the culture dishes. The second culture dish is your "control." It will show you what an air bacteria culture looks like without any chemical agents.
Store the dishes in a dark place like a closet where they will be undisturbed for a few days. After 3-7 days, take both culture dishes and carefully observe the bacteria growth in each dish, leaving the covers on. The bacteria will be visible in small, colored clusters. Take notes of your observations and make drawings. You could also answer the following questions. In the control culture, How much of the dish is covered with bacteria? In the sensitivity square test culture, Have the bacteria covered this dish to the same extent as the control culture? What effect have each of the chemicals had on the bacteria growth? Did a particular chemical kill the bacteria or just inhibit its growth?
Homemade Yogurt - Using Good BacteriaGenerally when people think of "bacteria," they think of harmful germs. However, not all forms of bacteria are bad!
You can enjoy a tasty product of good bacteria by making a batch of yogurt at home.
You'll need to use a starter (available at grocery or health food stores), or else one cup of plain, unflavored yogurt that has live cultures in it. (If it contains live cultures, it will say so on the container.)
Slowly heat four cups of milk until it is hot, but not boiling or scalding. The temperature should be around 95-120 degrees to kill some of the harmful bacteria. Cool slightly, until milk is warm, and then add one cup of active yogurt or the starter.
Put the mixture in a large bowl (or glass jars) and cover. Make sure that the bowl or jars are sterilized before using by either running them through the dishwasher or washing them with very hot water.
There are two different methods for culturing the yogurt mixture: You can put the covered bowl or jars into a clean plastic cooler, and fill the cooler with hot water to just below the top of the culture containers. With this method, you will need to occasionally refill the cooler with hot water, so that the temperature of the yogurt stays consistent. The other method is to wrap the containers in a heating pad and towels, setting the heating pad on low to medium heat.
Check the mixture after heating for 3 1/2 to 4 hours. It should be "set up," having a smooth, creamy consistency similar to store-bought yogurt. If the mixture is not set up yet, heat it for another 1-2 hours. When it is the right consistency, add some flavoring—such as vanilla extract, chocolate syrup, or berries—and store the yogurt in the refrigerator. It should keep for a couple of weeks. For safety, we suggest that you do not eat any yogurt that has separated or has a non-typical consistency.
For more experiment ideas and a step-by-step procedure (+ video), visit our Bacteria Science Project Guide.
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