Then, we grew bacteria on plates and learned an important lesson about ubiquitous bacteria and washing your hands.
First Daughter, showing her smile along with her bacteria. What fun! The colonies started growing noticeably within a few hours. After a few days, these were the plates we talked about the most:
For the plate above, we dipped q-tip in a bit of water and then wiped it across her a palm a few times. Her hands were not obviously dirty at this point, but it was late in the afternoon.
Then, she washed her hands (with regular soap and water, none of that antibacterial stuff around here) and we repeated the damp q-tip swipe for the "clean hand" plate.
Notice the difference? It was even more impressive early in the experiment, when the clean hand plate was actually still remarkably clean. All those bacteria growing and growing and growing...
I love this kind of activity. I used to pour plates and grow bacteria on a regular basis in college, so I was very excited. Even if you do not have a science background, though, this is an easy activity. You can buy a kit, or you can put together your own.
I bought Nutrient Agar Bottle - 200 Ml. and some plates. The directions are on the agar bottle; all you need is a microwave. I poured nine plates with this bottle.
- Make the agar following the instructions on the bottle.
- Use a oven mitt to hold the bottle so you can pour the plates while it's still hot. (This step should be done by an adult or responsible teen.)
- Be sure you place the top on the petri dishes as they cool and solidify. We let ours sit for a few hours and returned in the afternoon to add our bacteria.
- Leave one plate empty as a control (so only any bacteria that fell from the air as you were pouring would be present).
- Use a q-tip to present bacteria to each of the other plates. (Use a new q-tip every time.)
- Dampen the q-tip with water.
- Rub the q-tip back and forth a few times on the test surface.
- Gently rub the q-tip across the surface of the solid agar. If you want to get really fancy, you can see the "scientific" way to add bacteria to your plates here.
- Tape up the edges so the plate doesn't accidentally spill open. (I used masking tape.)
- Label the plates as you go. We used numbers (written on the masking tape on the side) and then wrote the master list in First Daughter's science notebook.
- Check your plates every few hours on the first day. (Hold them up to a light to see more clearly.) I asked First Daughter to sketch them.
- Do NOT open your plates. When you have observed them for a few days, throw them away. When those bacteria multiply in colonies, they can easily build up the kind of numbers that can make someone ill.
We didn't have a large number of plates and First Daughter is only in second grade, so mostly we were just seeing what happened. Here's the list she created:
- unwashed hand
- unwashed hand with a spot of antibiotic cream*
- washed hand
- inside cheek
- top of the compost lid
- bathroom sink
- temperature - store the plates at different temperatures (good for winter or summer but be wary of storing these in your refrigerator or freezer with food; you'd probably want some freezer quality plastic bags to keep them separate);
- light - constant light, constant darkness, alternating;
- kitchen counters or other surfaces after cleaning with different products;
- and all sorts of other interesting questions.
* If you want to use an antiobiotic cream, I recommend smearing some straight across the plate or on one whole side of the plate (using a new q-tip). We just dropped some in and the white glop of cream didn't really mix with the bacteria well enough to make any conclusions.
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