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Science Experiment: Which Surface Has the Most Germs? (Ewwww....)

We have been learning about bacteria, viruses, and germs as part of our Prairie Primer lessons for By the Shores of Silver Lake by Laura Ingalls Wilder. So we did an experiment.

We did not have any petri dishes or agar so I consulted the web and found an alternative. Our "dishes" were small, plastic drinking cups from Dollar General and our "agar" was 1 cup hot water, 2T clear gelatin, and 2T sugar all boiled and dissolved together. We poured the "agar" evenly into the cups held by an 8x8 baking dish, covered them with Press 'n Seal wrap, and put them in the refrigerator overnight to solidify the gel.

The next day we were ready to set up our experiment. We made a list of the surfaces we wanted to test for bacteria. Our choices were:

  • Brynne's mouth
  • Eli's mouth
  • Our dog Quincy's mouth
  • The kitchen counter
  • The dining room table
  • The toilet
  • The bathroom sink handle
  • The backdoor knob. 
We took a clean Q-tip and rubbed it on a surface, then gently rubbed the Q-tip all over the top of the "agar" in its labeled cup. The kids did this for each surface. Then we covered the cups back up with Press 'n Seal, put it in a black plastic bag, and put it on one of the dining room chairs that is closest to our furnace vent. (We had read that we needed dark and heat to grow the bacteria.) Then we waited.

In the meantime we made our hypotheses about which surfaces would have the most and least bacteria.
  • Eli- Most: Quincy mouth; Least: Eli mouth
  • Brynne- Most: Bathroom sink; Least: Quincy mouth
  • Mom- Most: Kitchen counter; Least: Toilet
  • Daddy chimed in on Instagram that he thought the dining room table would have the most bacteria.
We checked the progress on Day 2 and there was no change. We were going to check it on Day 4, but Brynne was sick with the flu and we didn't do any schoolwork that day. So we didn't get it checked until Day 6. When we took off the Press 'n Seal .... ewwwww!!!!

Daddy was right.

The top three bacteria-laden surfaces were:

Dining table, Kitchen counter, Toilet

The three least bacteria-laden surfaces were:

Bathroom sink, Eli's mouth, Brynne's mouth

Well, at least the children's mouths are low on bacteria! But, obviously I need to do a better job of cleaning (that's no big surprise ... especially on our dining room table. Yuck!) The doorknob and Quincy's mouth were right in the middle with a small bit of bacteria starting to grow.

What a fun and informational experiment! This is one I recommend for all ages. My kids also do Science Notebooking with their experiments, but for littles you could do it just for the gross factor. Older kids could throw in other variables, like amounts of light and levels of heat.


  1. I totally have to do this! I would have thought my table and kitchen counters would be okay, and door knobs and toilet worse!


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