Mucky Paws

Friday 16th January 2015

I've never understood people who wear their outdoor shoes in the house. Maybe they watch where they tread more than I do, or maybe I'm a bit OCD?

But then again, our dog has free run of the house and he regularly treads all sorts of things into the carpet, the sofa, and our bed.

So I thought it would be interesting to see what has the most bacteria - his feet, my shoes or my hands.

I did a science degree a long time ago, so I dusted off My First Book Of Science Experiments and tried to remember how it all works.


My hypothesis is that the dog's feet are dirtier than my shoes.


I bought 10 petri dishes (£5.50) and a pack of nutrient agar (£2.49) from eBay (I love eBay), mixed up the agar with water and boiled it for 25 minutes to sterilise it. In the most scientific way I could, I placed five samples onto the dishes, each one twice for extra Science Points™

  1. Stanley's front paws (labelled F. Paw 1 & 2)
  2. Stanley's back paws (R. Paw 1  2)
  3. My hands (Hand 1 & 2)
  4. My shoes (Shoe 1 & 2)
  5. Control, with nothing on it (Control 1 & 2)

I'd been washing my hands regularly and wearing gloves while I made up the plates, so my hands were a bit too clean for a good sample. To make the results more interesting I brushed my hands through my hair, round the back of my ears and then shoved them down the front of my trousers for good measure.

The control samples had nothing placed onto them, they were just sealed up like the other samples for even more Science Points™. It's been at least 15 years since I've done any kind of real science, so I expected the control samples to be the most heavily contaminated!

I piled up the dishes into two stacks of five, then placed them in my highly advanced incubator (on top of the radiator in the guest bedroom) and left them for five days.


Five days later, and the results are in:

The dog's feet harbour more bacteria and fungi than my shoes, but they're pretty similar.

There's quite a lot of variation between the samples, so I wouldn't call this a good result. The thing I'm most impressed by is that the control plates are clean (the dots in the photos are bubbles in the agar), which means my aseptic technique is better than I thought it was.

Most of what you can see in the bottom left of the Shoe 1 sample is where I broke the surface of the agar, not bacterial colonies. And even though I tried to make my hands dirty, they're the cleanest sample of all.

[Edit: It's been pointed out by somebody with more Science Points™ than me that me that not all bacteria and fungi are bad. Indeed you've got more bacterial cells in you than your own cells.]


  1. We should probably stop the dog sleeping on our bed, or at least clean his feet.
  2. I can still do science!


What would you test?

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