Paws and Petri Dishes 

An at-home science activity that tests the cleanliness of your pet’s paws

Discovery Place Kids Rockingham

If you have ever visited the Explore More Me lab at Discovery Place Science, you already know there are lots of microorganisms that call the human body home. These bacteria, fungi and invertebrates are not necessarily bad for us – in fact, many, like the bacteria in our guts, help us be healthy!

Your pets also carry an ecosystem of critters with them wherever they go. Bacteria, fungi and other causative factors of odors are normally present on your pet’s skin from day to day. Bacteria is everywhere, even lodged in your pet’s paws.

Have you ever taken a whiff of your pet’s feet and smelled corn chips? “Fritos feet,” or “Fritos paws,” is a common report from pet owners and can be easily explained by microbiology. Not only do pets sweat through their paws, but they also experience growth of the bacteria Proteus and Pseudomonas there. The growth of these bacteria, commonly found in soil and feces, are what causes the corn chip smell. If your pet takes frequent visits outside more than likely these bacteria are living on their paws.

Let’s do some microbiology at home to get a firsthand look at the organisms living on our pet’s paws. First, we will make our very own agar plates using household products. After the plates have solidified, we can take our pet’s paws and press them onto the agar or swab them with a clean Q-tip. After some days at room temperature, we should start to see some bacteria grow.

Just how clean are your pet’s paws? Let’s find out!


This recipe makes up to two Petri dishes.

  • 1 cup water
  • 1 T agar-agar (OR 1 ½ packages gelatin, which is about 1 ½ oz. or 12g)
  • 1 bouillon cube (or 1 tsp. granules)
  • 2 tsp. sugar
  • Saucepan for boiling mixtures
  • Spoon
  • Petri dishes (or clean plastic containers with lids)
  • Marker to label Petri dishes
  • Cotton swab (optional)
  • Tape (optional)
  • Some type of incubator oven. This could be a warm spot behind the fridge, near a heater or a box with a desk lamp inside or on top. (optional)


1. Wash your hands!

2. Pour the water into the saucepan and bring to the boil.

3. Add beef stock powder, sugar and gelatin to the boiling water and stir for 1 minute until all ingredients have dissolved.

4. Be sure to cover the mixture when possible to avoid contamination from bacteria in the air.

5. Cool your new agar mixture slightly, for about 10 minutes. The mixture needs to be still hot to avoid the gelatin solidifying in the saucepan.

6. Take the lid off of the Petri dishes or clean plastic containers and have an adult fill the containers halfway with the hot mixtures. Only remove the lids of the containers when you are ready to pour the hot mixture to avoid contamination.

7. Quickly place the lid back on the Petri dishes or containers and allow the gelatin to set in the fridge for a minimum of 2 hours. They can be stored in the fridge for up to two days.

8. Be sure not to touch the agar with your hands, remember we want our Petri dishes as sterile as possible!

Now that we have our Petri dishes, it’s time to collect and grow our bacteria. First, wash your hands again. Remember, bacteria is everywhere! Next, it’s time to collect and grow your bacteria (or fungi) on the agar Petri dishes. Here’s is how you do it:

1. Get your treats ready and take an imprint or sample from your pet’s paw. Samples can be taken by lightly pressing the pads of your pet’s paws to the agar. Samples can also be taken by taking a clean cotton swab and collecting a sample from your pet’s paws. Rub the cotton swab lightly across the agar plate in a zig-zag motion. Whichever method you choose to collect your bacteria be sure not to puncture the agar.

2. Quickly, put the lids back on the Petri dishes, label them, tape them closed and place them upside down in your incubator (if you have one) for 1-2 days. If you don’t have an incubator, leave the plates at room temperature for 3-5 days.

3. After the incubation period, check out the bacteria growing on your petri dishes! As a precaution, do not open your sealed Petri dishes. Dispose of the entire sealed plate in the bin when you are finished.

Below are some plates of Pseudomonas and Proteus. Do these look like anything you grew on your plate?

Pseudomonas aeruginosa

Pseudomonas aeruginosa known to turn a bluish green hue on an agar plate!

Proteus mirabilis

Proteus mirabilis can grow in single colonies but also can be spotted by a “swarming motility”. Bacteria that swarms can move all around the agar plate and will make some noticeable rings to let us know it is moving.


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