In this experiment you will assess how easy it is to

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In this experiment, you will assess how easy it is to transfer a microorganism with your hands and how to prevent this transmission by handwashing. Baking yeast (Saccharomyces cerevisiae) is a unicellular microorganism that does not cause disease, and it can serve as an effective model for how microorganisms can be transferred. Note: This experiment should be started with unwashed hands. Notes About Working With Agar Plates • Use the agar sparingly. You need only pour enough to coat the bottom of the petri dish. • Prepared and fully gelled agar dishes should be stored upside-down in the refrigerator until use. This will prevent condensation from disrupting the growing surface as well as any aerial contamination. • Before inoculating, let agar plate warm up to room temperature (about 1 hour). Figure 2: Image of yeast colonies growing on culture media. Colonies should be small, round, and white to slightly yellow in color. Fuzzy- appearing growth indicates mold contamination and should not be counted. • After inoculating, replace the cover on the dish, seal with Parafilm®, and store upside- down in a consistently warm location (not to exceed 37.7 °C or 100 °F). • You should see growth within a few days (Figure 2). If you do not see growth, your plates may be incubated at a temperature too cold for microbial growth. • Before disposing of plates, kill the microorganisms by pouring bleach solution onto the agar surfaces and let sit for at least 20 minutes. PROCEDURE 1. If you are keeping a lab notebook, record the date, time, and experiment title on a fresh page before you begin. 2. Loosen or remove the cap on the nutrient agar bottle. 3. Place the bottle in a microwave. You will need to remove the bottle from the microwave and swirl the contents every 10 seconds to evenly distribute the heat. If you do not have a microwave, place the bottle
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in a heat-safe bowl, create a water bath by pouring boiling water into the bowl (around the bottle), and heat until the entire bottle of agar is liquefied. Note: If you notice the agar boiling over, STOP the microwave and let the bottle cool down before handling. Hot agar can violently explode out of the bottle if heated too quickly and/or shaken. Once boiling has stopped, use a hot pad to protect your hands and remove the bottle from the microwave. Use caution when removing the bottle from the microwave as it will be HOT!
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  • Fall '19
  • Agar plate, Petri dish, Agar

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