Handwashing Lesson (Homemade Petri Dishes)


With all of the bugs going around at this time of year, I thought it would be a great time to remind your kids why they need to wash their hands.  Culturing microbes (bacteria and fungi) on petri dishes lets them test different surfaces for microbes and grow their own germs.  Even very young children will have fun helping with the Q-tips and seeing what grows in their microbial zoo.  It’s fun, easy, and you probably have what you need in your kitchen cupboard:

IMG_3658disposable containers to grow bacteria in (see below),  beef bouillon cubes or granules, plain gelatin, water, sugar and Q-tips.

For containers, you can use foil muffin tins, clear plasticware with lids, or real petri dishes to grow fungi and some bacteria.  We’re going to use clear deli containers so that we can recycle while we learn.  (They look like they will be heat-resistant enough to pour warm agar into.)  You’ll start by making microbial growth medium (or germ food, as we like to call it.)  Help your child mix together a little less than 1 cup water, one package gelatin, one bouillon cube (or 1 tsp. granules), and 2 tsp. sugar.  The next step is for an adult to do.  Bring the mixture to a boil on the stove, stirring constantly, or boil in the microwave, stirring at one minute intervals and watching carefully.  Don’t stir after the liquid boils.  Remove the boiling liquid from heat and cover it with aluminum foil.  Let the growth medium cool for about fifteen minutes.


Pour the medium carefully into clean containers, until 1/3 to 1/2 full.  Loosely place lids or foil over containers and allow dishes to cool completely.  The agar should make the growth media hard like jello.  When the agar has hardened, store the plates in a cool place, like a refrigerator, before using.  Plates should be used in 2-3 days.  When you are working with the plates, try to keep the lids on whenever possible, so that they are not contaminated by the air.  If you’re planning to use muffin tins, simply place them in a muffin pan, fill them with agar, and when they’re cool, put them in individual zip-lock baggies.  With other containers, put the lids on tightly once the plates harden. 

When the plates have hardened  and you’re ready swab, shake the condensation off the lids of the containers and put them back on.  Then, help your child draw a grid of four sections on the bottom of the plate with permanent marker. (If you are using muffin tins, you’ll just label each bag with the surface you are checking.)  Ask your child which surfaces they’d like to test.  It’s always fun to label one section of the grid “fingerprint” to let them see what grows when they touch their finger to the plate.  Label each section with the surface they want to test.   Be sure to label the bottom of the plate since the lid will move.  You should be able to see through the agar to see your lines and your writing.  If you want to, you can label a separate plate for each surface, but we had three kids and three plates, so we made sections.  TV remotes, kitchen sinks, computer keyboard, doorknobs and piano keys are great surfaces to check.  See the photo at the top of this post for a better picture of how your plate might look.


Now comes the fun part.  Have your child rub a clean Q-tip around on the surface they want to test.  Then, remove the lid from their plate and help them rub the Q-tip across the section of the plate labeled for that surface.  If they are gentle, the agar shouldn’t break.  If it does, it’s no big deal.  When you have finished, set the plates on a flat surface with thier lids loosely set on top (do not invert them, as I first suggested.)  I set our plates on a countertop where they won’t be in the way.  Have your children check their plates every day, and soon they will observes colonies of different shapes, sizes and colors starting to grow.


They will mostly see fungi (molds), but they may also see some tiny clear or white spots that are colonies formed by millions of bacteria.  Your child can record and draw how their plates look in their science notebooks.  Older children can keep track of how long it takes things to grow and the shapes, sizes and colors of the microbial colonies that grow on their plates.  If they want to learn more about microbes, help them search for the words fungi and bacteria on the website cybersleuthkids.com and it will give them some great links to microbiology websites.  Tell your children that microbes are everywhere, but that very few of them are harmful, and that many of them are essential for good health.


Have your children wash their hands after handling the plates, and throw the plates  away when you are done.  Remind them that if they wash their hands with regular hand soap for the length of time that it takes to say the ABCs, they’ll remove most of the harmful bacteria from their hands.  (For adults, a severe side effect of this experiment is the sudden urge to disinfect computer keyboards and remote controls.)  

Here’s what grew on our plate: The large, fuzzy colonies are fungi and the small, whitish ones are probably bacteria.  The grid with the most fungi was cultured from our piano keys.  The one with both fungi and bacterial colonies visible was cultured from our bathroom sink.  One grid has mostly small, white bacterial colonies and was cultured from a water-glass my son drank from.  The fingerprint grid has only a single fungal spot.  My daughter must have washed her hands before touching it!  Our other two plates were pushed too close to the under-counter lights in our kitchen and the agar melted, so we threw them away.  I’m going to clean off my piano keys now!

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2 Responses to “Handwashing Lesson (Homemade Petri Dishes)”

  1. Unplanned Cooking Says:

    This is a great experiment. With all the talk of germs in school right now, my children would find this very interesting.

  2. Chinese Dishes Says:

    Hello, great read. I just now stumbled on your website and I am already a fan. 🙂

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