Adventures in Wonderlab

Adventures in Wonderlab

Fun science adventures for all ages!

Do I Really Have to Wash My Hands? An Experiment with Hand Washing and Growing Bacteria

We finally got the chance to test out our new agar plates and do an experiment growing bacteria. Since it’s that special time of year again- when all of the bad germs try to make their way into our bodies and make us sick- we decided to do a hand washing experiment. How effective is hand washing, anyway?


Agar Plates and Growing Bacteria

I purchased some luria broth agar plates, which are used to grow environmental bacteria, including E. coli. Other types of agar plates are used to grow pathogenic bacteria- I did not want to grow any of those in our home! I should also note here that only bacteria can be grown on agar plates – no viruses.

Petri dishes

Heating lamp and thermometer on the left (for our incubator), and agar plates are stacked on the right.


When it is time to grow the bacteria, we placed our agar plates in an upside-down position in a homemade incubator with a temperature between 85 and 100 degrees F. For our homemade incubator, we used a plastic bin, a thermometer, and a heating light bulb as a heat source.

Homemade incubator

Growing bacteria

After 1-2 days in the incubator, we watched for the appearance of small bacterial colonies (usually white or yellow dots) on the surface of the agar plates. You need millions of bacteria in one spot just to see one dot on the agar plate.

The Experiment

I sent both kids outside to play for an hour. I called them back inside one at a time, so I could closely instruct them for our hand washing experiment. My son came in first – I took his hand, and gently pressed his fingers into one agar plate. This will be known as the “dirty hand” plate. Then, he washed his hands with soap as usual – like a child in a huge hurry – I timed him at 10 seconds total. Next, he dried his hands with a clean paper towel, and pressed his fingers into a second agar plate, the “clean hand” plate. We repeated this process with my daughter. She is a more thorough hand-washer, and her time was 30 seconds total of scrubbing and rinsing.


I fully expected to grow lots of bacteria on the “dirty hand” plates, and grow little to no bacterial colonies on the “clean hand” plates.


Though we did grow a lot of bacteria on the “dirty hand” plates, I was pretty surprised how much bacteria we were able to grow on the “clean hand” plates. And my daughter (S.), the thorough hand-washer, actually grew more colonies from on her “clean hand” plate than my son (J.), the fastest hand-washer in town.

Bacterial colonies

“Dirty hand” plates are on the left, “clean hand” plates are on the right


We repeated this experiment, and got similar results the following time.

Bacterial colonies

“Dirty hand” plates are on the left, “Clean hand” plates are on the right



Yes, wash your hands! It certainly decreases the amount of bacteria on them. Perhaps the length of time spent scrubbing isn’t as important as I thought, but I am sticking by the 20-second scrubbing rule. Like I said, this experiment did not test for all viruses, or all types of bacteria. Might as well be safe.

bacterial colonies

A close-up view of the difference hand washing can make on number of bacterial colonies grown. “Dirty hand” on the left, “Clean hand” on the right.

Also, we do not use anti-bacterial soap at our house. Just regular soap. We stay very healthy at our house year-round, so I see no reason to try to kill all the bacteria in our home. Don’t forget, bacteria can be very good, too!

That being said, there are times when you do want to kill all the bacteria in your home! For example, if someone has a compromised immune system, if someone in the family is ill, etc. In those cases, we do keep hand sanitizers around the house.

For a fun at-home experiment with your kids, try the “scaring the germs away” experiment we did last year around this time. A great visual example of how soap works to scare germs off of our hands!

These are the products I used for this experiment:

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