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How many germ-carrying droplets can cross cloth?
Credit: CITIZEN SCIENCE: Eichler, Hopperton, Alava, Pereira, Ahmed, Kozlakidis, Ilic and Rodriguez-Palacios. This is an open-access
article distributed under the terms of the Creative Commons Attribution License (CC BY).
Recommended Grade Level(s):
Appropriate for: Middle and High School
Time Requirements:
48 hours
Teaching Topics & Concepts:
A face mask decreases the transport of droplets carrying bacteria
and viruses by >98%.
Background:
By using a spray bottle filled with a "germ solution" (diluted yogurt, soil) and
catching the germs on gelatin growth plates, the goal of the experiment is to de-
termine how many macroscopic and microscopic droplets containing germs can cross a face cover.
Materials:
Germ growth plates and materials (advanced, prepare 1 day ahead)
• 1 spray bottle
• 1 oz. yogurt, soil, or other cultured food to make a germ-simulating solution
• 2 empty cereal boxes
• 6 shallow plastic containers at least 2" diameter, or foil cupcake liners
• 1 tsp sugar
• 1 cube beef bouillon (chicken is ok)
• 2 pack plain gelatin, ¼ ounce each
• 1 cup water
• Microwave and a 12 oz. glass container, or pot with lid to use on stovetop
• Clear zip-lock bags
• Clean face cover
Make your own nutrient agar: https://www.instructables.com/id/Homemade-Nutrient-Agar/
Procedure:
Part A.
The day before—Prepare the gelatin germ growth plates:
1. Mix 1 cup of water, 1 teaspoon sugar, and 1 beef broth bouillon cube in a microwaveable measuring container in the
microwave for 2 minutes. Stir well when done, then heat for 2 more minutes. Leave the mixture inside the microwave to
cool about 5 minutes.
2. Alternatively, over medium-low heat bring the water, sugar and bouillon to a low boil while stirring, boil for at
least 2 minutes. Cover with a tight-fitting lid and turn off heat. Allow the mixture to cool for several minutes.
3. Put on your face cover and re-wash your hands to avoid contaminating your plates.
4. Slowly add 2 packets of gelatin powder while stirring.
5. Then carefully pour the mixture into shallow round containers (for example, well-cleaned, recycled yogurt container lids)
or foil cupcake tins about ½" (1cm) deep. You should get at least 6 growth plates. Immediately place poured gelatin plates
in a covered container or plastic bag and leave unsealed to allow moisture to escape.
Place in a cool location to solidify overnight (a cool oven works well). Plates must be cool prior to testing. DO NOT touch
the prepared gelatin with your fingers (this could contaminate them!). Prepare at least 4 gelatin growth plates for Part D. You
may want 2 extra plates for each additional type of cloth you want to test. Store in a sealed zip-top bag until use.
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