Let’s talk money.  Specifically germs on money.  The lesson plans that hit my store this week focus on money.  We’ll cover math, literacy, and social studies in the activity packs, but you know I can’t leave out science! And I don’t know about you, but when I think money and science, I think germs!    We’re going to test money and see exactly how germy it is by growing bacterial samples we’ve collected from bills and coins.

Click here to watch a complete video of this experiment, including additional science info.

If you’re like me, you’ve cringed watching your kids put coins in their mouths or eat straight after touching bills.  I’m not a germaphobe, but I know how many hands money touches. And it’s not that the boys were ignoring me when I told them ix-nay on the coins in mouth, it’s that the concept of germs was totally foreign to them.  I mean, you can’t see them, taste them, or smell them, so do germs even really exist?  

One way to convince those beautiful brains that germs really do exist is by showing them.  Seeing is believing, right?

Let’s take some bacterial samples, help them grow, and then present evidence of germs’ existence! 

Don’t be intimidated by the idea of collecting and growing samples, it’s super easy! 

Here are step by step instructions:

Supplies:

  • An agar plate kit
    • The exact kit I used is out of stock, which is a bummer because it was a great kit. But, this kit looks good and it has over 1,000 good reviews.
  • Gloves
    • You want to be sure that all the bacteria on your plates actually come from your source, not from your hands or your kiddo’s hands.  Glove up!
  • Distilled Water
    • We’re using distilled water because we want to make sure all our bacteria comes from the money, not from outside sources.
  • A permanent marker
    • We’ll need to label the bottom of our plates so we know where our samples came from.
  • Money
    • Whatever bills and coins you’d like to use.  I did one of each coin and bill, up to $20. You do you boo!
  • An infrared thermometer
    • Like this.  Sadly, your normal, human temp thermometer won’t work as the ideal bacterial temperature is too low to register on a human thermometer. You’ll want to be able to check the temperature of your bacteria quickly and easily.
  • OPTIONAL-  a Cardboard Box  and lamp to create a warm space

Step 1: Label the Bottom of the Plates

Image shows gloved hands holding an agar plate with one cent and one dollar written on it.  There are several different American bills in the background.
You can see here that I’ve labeled this plate as 1 cent and 1 dollar.

Each plate has a lid and a bottom portion filled with agar.  Flip the plate over so you’re writing on the bottom of the plate and label which coin or bill you’ll be sampling from on this plate.  You can easily put two samples on the plate, just draw a line across the bottom so you’ll know where to stop your sample.

Step 2: Collect Your Samples

Photo shows a set of gloved hands collecting a bacterial sample from a $1 bill with a qtip. There are three other bills in the background of the shot.
Run a sterile swab dipped in distilled water over a bill or coin. That’s it!

Sounds so official, but really, it’s just rubbing a damp cotton swab over your money!  

  • Put on your gloves, 
  • Open the sterile swab packet (note, do not just use q-tips for this.  They aren’t sterile.) 
  • Dip the swab in your distilled water.  The moist swab collects bacteria better than a dry swab.  
  • Rub the moist swab all over the coin or bill. Roll the swab around to coat the entire swab. Get both sides of the coin or bill.  

That’s it!  You’ve collected a sample!

Step 3: Transfer Your Bacteria

Image shows gloved hands running a sterile swab over an agar plate.  There are bills in the background.
Run your swab gently over the agar plate.

Now we have to put the bacteria somewhere they can grow.

  • Take the lid off your agar plate.  
  • Paying attention to your labels, rub your swab on the plate.
    • Make sure you rub the correct area.  
  • Roll the swab around as you rub to get all the bacteria off.  Be careful not to puncture the agar plate (kids might need help to understand the amount of pressure needed). If you do puncture it, it’s not the end of the world!

Step 4: Rinse and Repeat

Collect and transfer samples from each of the coins or bills.

Step 5: Incubate Your Plates

The bacteria needs to be at 80-100 degrees F to grow.  Unless you keep your house pretty warm, you’ll likely need to create a warm space for your plates.

I’ll tell you what I did and why you might not want to do it.  Then I’ll give you some other options.  

I created a makeshift incubator out of a cardboard box and a candle warmer.   I punched a hole in the side of my box just large enough to allow an electrical plug through.  Then, I put a candle warmer in the box and ran the cord out the side.  The warmer doesn’t get very hot, in fact I can touch it with my bare hand.  But, I spent 5 days convinced my house was going to burn down at any second.  If you’re more of a dare devil than me, this actually worked really well.  It took 3 years off my life from worry, but it worked really well!  

If you’re not a dare devil, or you value your sleep, here are a few other options for warming those plates:

  • Put them on top of the fridge
    • A lab savvy friend of mine told me that’s what she does to incubate plates at home away from her lab.
  • Build this incubator.  
    • If you have a lamp and a couple yogurt tubs, this would be a great choice!
  • Find a warm spot in your house. Do you have a cabinet that holds some electronics?  It probably gets warm in there.  You could tuck the plates in there to incubate.  

Do not put the plates in direct sunlight.  If you do need to put them in sunlight, you can wrap them up in foil to prevent the light from getting in.  UV can kill bacteria and after all our collecting and transferring, we don’t want to kill our germs!

Place your plates lid side down.  This prevents any condensation that forms from dripping on your agar and contaminating things.

Place all the plates evenly around your heat source so they all grow at roughly the same rate.

Step 6: Monitor Your Plates

  • Check your plates a couple times a day to make sure the temperature is in that 80-100 F range.  Adjust plates closer to or further from the heat source to keep them in range.  
  • You’ll start to see growth sometime in the 12-48 hour range.  Don’t let the plates grow for more than 5 days.  

Step 7: Check Out What You Grew

   As soon as you’re happy with the amount of bacterial colonies on your plates, pull them out of your “incubator” and make observations!  Note: Leave the lids on as you observe.  You don’t want to breathe in whatever bacteria we’ve grown and multiplied!  You might want to tape the lids on just to avoid any temptation for little hands to open lids. 

Questions to ask your kids as they observe:

  • Count the colonies, which bill had the most?  Why do you think that is?
  • Which plate had the least?  Why do you think it had fewer colonies than the others?
  • Flip the plates over and examine the tops of the colonies. Why do they look different from the bottoms?
  • There are probably several different types of colonies on the plates. Why is that? Why do different bacteria grow on different bills?
  • How do you think the germs got on the money?  
  • Now that we know there’s bacteria on money, what does that mean about other things we touch?  

If you’re interested in adding a math component to your experiment, you can graph the number of colonies on each bill or coin.  Download a blank bar graph here. Again, I cover this experiment and more in my YouTube video, including how to graph it and an analysis of the results.  You and your kids can watch it here.

If you’re interested in doing more money activities with your kids, including money math, budgeting/financial literacy, reading and writing, check out my complete money lessons here.  

That’s it!  You’ve now collected, grown, and analyzed bacterial samples. So fancy!  Hopefully you and your kids got great results and learned something new.  And maybe, just maybe your kids will think before they put something gross in their mouths!