Growing Bacteria in Petri Dishes Take samples and see what will grow in an agar Petri dish. email to friend
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1. You'll need a clean, microwave-safe container (quart-size bowl works great) to mix and heat the agar with water. These mixing proportions make enough nutrient agar to prepare two halves of the Petri dish. Mix 1/2 teaspoon agar (about 1.2 grams) with 1/4 cup (60 mL) of hot water and stir. Bring this mixture to a boil for one minute to completely dissolve the agar. CAUTION: Adult supervision is required to boil water. If you are using the microwave oven to boil the mixture, be careful not to let the solution boil over. The mixture should be clear “ no particles floating around in the solution. Allow the mixture to cool for 3 to 5 minutes before moving onto the next step.
2. Carefully pour equal amounts of the warm agar nutrient solution into both halves of the Petri dish. Loosely cover each Petri dish with plastic wrap and allow the solution to cool and harden for at least an hour.
3. It's time to collect some bacteria on the end of a cotton swab. The classic test is to roll a clean cotton swab in your mouth and then to lightly draw a squiggle with it on the gelled agar. However, many people like to test something even more gross like the keys on your computer or the television remote control. Unless someone recently clean the buttons on the TV remote, you're in for some real YUCK in a few days.
4. Since you poured agar into each half of the Petri dish, you have two places to grow bacteria. Consider all of your options below (or come up with your own) to collect samples. You might want to collect a sample from a computer keyboard for one half of the Petri dish and collect a sample from a door handle for the other half. Remember, you must use clean cotton swabs for each sample. In order to get a good sample collection, dampen the end of the cotton swab with water. Be sure to wipe the end of the cotton swab all over the surface to be test “ cover the end of the swab with invisible bacteria! Things that you might want to test Door handles, your hands, under your fingernails, your mouth, the top of a desk, computer keyboard, remote control, pencil or a pen, area around a bathroom sink, fax machine, calculator or your favorite toy.
5. Pull the plastic wrap covering away from the Petri dish and LIGHTLY draw a squiggly line in the agar with the end of the cotton swab. Cover the Petri dish with plastic wrap, use a piece of paper or tape to label the dish with the name of the item you tested and place the Petri dish in a safe, dark place to let the bacteria grow.
6. Here's a clever test try placing a drop (no more) of hand sanitizing gel in the middle of one of your squiggles. Your hypothesis might be that the antibacterial chemical in hand sanitizer will keep any bacteria from growing. We'll see if you're right.
7. Cover the Petri dish with plastic wrap, use a piece of paper or tape to label the dish with the name of the item you tested. Place the plates in a warm dark place to grow - not too warm, but anything up to about 98 degrees F (37 degrees C) should be fine. In a short time, you'll be greeted by an amazing variety of bacteria, molds, and fungi. You should continue to see more and larger colonies for the next few days, but you should not see any growth where the disinfectants (hand sanitizers) are. You might even see a "halo" around each spot where you placed the hand sanitizer. This halo is called the "kill zone" - measure and compare the size of the kill zone to determine effectiveness of different antibacterial agents.

Golly Mom is right! It is important to wash your hands whenever you can! Remember... Do not open the plates once things begin to grow. You could be culturing a pathogen.