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How Clean Are Your Hands? Using Petri Dishes to See Bacteria and Mold

I recently blogged about a microscope study we did. The kids were super interested in learning about bacteria after that! They wanted to find bacteria in the pond water, and we did, but that didn’t hold their attention long at all. Once I mentioned we could SEE the bacteria growing on our hands (or anywhere, really) with a petri dish, that’s all they could think about! So, we jumped right in.

The first thing we did was read up on using petri dishes. We learned that growing bacteria can be dangerous. In the small doses on your hands, it’s less likely to cause illnesses and such, but in the petri dish, it becomes a hazard. Some websites suggestedΒ putting a piece of tape to hold the dish shut, but with such small children and the possibility of mold and other harmful bacteria growing, I put them straight into Ziplock baggies and instructed them not to ever open them. We printed a mold and bacteria identification sheet from here, and then we obtained our samples. (AKA rubbed wet q-tips on their grubby little hands.)

swab and dish

Once we rubbed for fifteen seconds on their hand, we rubbed the invisible bacteria onto the agar in the dish. We rubbed each one for ten seconds spinning the q-tip to make sure we got all of the “yuckies” transferred. I bought pre-made dishes from Amazon and here’s the link to the ones I bought.Β Making your own dishes seems to be more economical so I may get stuff to do it, and it’s similar to making jello. The kids really like to see bacteria and mold grow, so I have a feeling we’ll be needing more soon.

The kids made an incubator to put the dishes in to keep them warm. They used an old wooden craft box that no longer has supplies in it, a lamp (my pretty laundry room one), and a piece of dark colored fabric. They laid the dishes into the box, turned the lamp on, and covered it with the fabric. It’s a fifteen-watt bulb, so I wasn’t concerned about it overheating.

After 24 hours there wasn’t much change at all to the dishes. There was some condensation on the lids but other than that they were still “clean.”

The magic started happening about 36 hours into incubation. The kids were so excited to see the nearly overnight transformation.

36 hours in

After 48 hours there wasn’t much of a difference in the growth. It was growing in thickness, but there weren’t any new growths. Except in Ellas, she had some type of fungus starting to grow. At this point, they were all starting to stink, and I read in multipleΒ places that after about two days it was time to throw them out, so they don’t become harmful. So we took one last picture and threw them out.

When we swabbed our hands, we also set out a few different foods to see how mold grows on them and then left them out of the incubator. We talked about how giving the bacteria a good place to live would make it grow faster, but leaving things out at room temperature can cause it to grow more slowly. We left the items exposed to the air for about 30 minutes before placing them in the bags.

bread, kiwi, cheese, cucumber
One week at room temp.

They’re still out on the counter right now, rotting away. Yum. The white bread is the only thing that doesn’t seem to be changing. Which, isn’t surprising. Β I’ll keep everyone posted on Instagram, so follow along!

Does your family have a favorite gross science experiment?

How Clean Are Your Hands- Using Petri Dishes to See Bacteria and Mold (1)

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