Agar is a substance from the cell walls of red algae used in making petri dishes or "agar plates." Agar is a firm gelatinous substance at room temperature which is not broken down by the bacteria, making it an ideal substrate for culturing and observing the organisms. Although agar is the preferred petri plate, other ingredients such as gelatin can be used when no agar is available. You can make your own substitute agar plates at home out of common kitchen ingredients.
Wash your hands, your counter and all dishes you will be using thoroughly. Although you will not be able to get totally sterile conditions, you should try to be as careful as possible to avoid introducing germs to your petri dishes.
Mix 4 cups of cold water with 4 envelopes of unflavored gelatin in a saucepan. Stir in 8 tsp. of sugar and 4 beef bouillon cubes.
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Heat the saucepan on medium low heat while continuously stirring it.
Turn off the heat when the mixture boils and allow it to cool for 3-5 minutes. Fill sterile petri dishes 1/3 full of the mixture until you have used it up. If you do not have sterile petri dishes, place use aluminum cupcake holders in a cupcake tray and fill them about 1/3 full of the liquid.
Place the agar plates in the refrigerator for 2-3 hours to allow the gelatin to cool and firm.
Cover your agar plates. If you are using cupcake cups, cover the whole pan with plastic wrap or put each cup in a separate food storage bag. If you are using petri dishes, cover each with its top lid. Use your agar plates within 3 days.
Avoid touching, breathing on or otherwise coming into contact with any of your agar plates, since this can introduce bacteria, contaminating the plate.