- 1). Pour 100 milliliters of distilled water into a beaker and bring to a boil.
- 2). Measure 2.3 grams of agar into the water, stirring with a glass rod until the powder has dissolved. Then sterilize the agar. You can use an autoclave if your lab has one. Otherwise, a microwave should work. Wait until it is cool enough that you can pick up the agar.
- 3). Add agar to the petri dish until a layer covers the bottom. Wait until the agar is no longer liquid. Then, run the cotton swab over about a square inch of the food item.
- 4). Roll the end of the swab around the agar on the petri dish. Make sure all of the swab touches the agar multiple times.
- 5). Place the petri dish in an incubator or oven at 100 degrees F for up to 24 hours.
- 6). Set the dish under a microscope and count the bacteria colonies that have appeared. While the colonies are now larger than they were a day ago, this is the number of colonies that were on the section of the food item that you tested.
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