Introduction: Grow Bacteria From Gross Stuff!

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Ever had that hankering to grow millions of something? With a little swab from the inside of your mouth or the back of a toilet seat, you'll have a veritable bacteria farm empire in no time at all. Ever wanted to prove that a dog's mouth is cleaner than Ted from accounting's desk? Now is the time to do it with science!

Making agar agar plates for bacteria is a great way to (1) Grow and learn about the microorganisms and cultures for sprouting young biologists (read: students!), (2) Get to practice some kitchen chemistry, (3) Prove that the world is just as gross (and awesome!) as you thought it was, and (4) You get to say "agar agar" lots!

Let's make a bacteria farm!

  • What: Growing Bacteria From Gross Stuff
  • Concepts: biology, microbiology, cell life, bacteria, fungi
  • Materials:
    • Petri dishes (top and bottom)
    • Q-tips
    • White sugar
    • Bouillon Cubes (available at any most groceries)
    • Agar Agar (East Asian grocery stores, also some large general ones)
    • Measuring Cup
  • Tools:
    • Stove or way to boil water

Let's dive in and get cultured!

Step 1: Mark Your Dish

Start off easy by dividing your petri dish in to several quadrants where we'll test different swabs. Write on the bottom of petri dish in Sharpie. I chose to do symbols so I could make a key and didn't have to write backwards, as we'll mostly be viewing it from the other side.

Step 2: The Bacteria Food Mix

Time to make something the bacteria culture we'll be talking about for weeks!

None of this recipe has to be exact, but here are good ratios to start with. For a small recipe (about 4 plates), start off with boiling ~ 200mL of water. You want to get it truly hot hot hot. Mix in the teaspoons of agar agar, stirring to make sure it dissolves completely. This is super important in making it gel later. Then add in your sugar (for easy bacteria carbs) and bouillon (for bacteria protein), and mix it all together.

IMPORTANT NOTE: The agar agar really needs to dissolve completely, otherwise you end up with pudding at the end. We're going for a tough gel. If you need, you can mix all of this on a stovetop while the water is boiling to ensure it reaches that high temperature necessary.

Step 3: Pour and Refrigerate

Pour your food mix into the bottom of your petri dish so it is about 1/4" or so deep. Place the lid back on to avoid contaminations, and then place them in the fridge to make the cooling go faster. Leaving them at room temperature is okay, too, it will just go slower.

Step 4: Swab the Decks!

Time to get gross! Start with writing a key as to what the different cultures are in your bacteria farm. I chose to do my mouth, a dog's mouth (thank you Cloude!), the kitchen top, and a toilet. I truly have no idea which will be the dirtiest.

For each one, use a fresh Q-tip, swab as best as you can, and then rub it on the gel in the corresponding quadrant of your bacteria plate. Make sure to replace the lid each time to minimize contact with the air around (there's a lot of stuff floating around).

If you're looking for swabbing ideas, here are a couple other ones:

  • Mouths before and after using mouthwash
  • Surfaces before and after cleaning
  • Compost Bins
  • Trash Cans
  • Doorknobs
  • Hands before and after washing
  • Ted from accounting
  • Armpits
  • Ted from accounting's armpits

Step 5: Replace Lid and Tape

Cap your petri dish and you can add some tape to keep the lid on while we wait for our bacteria colonies to grow. Keep in a warm environment for maximum growing potential, try to leave it relatively undisturbed. Oh, I can't wait to see what grows!

Step 6: Bacteria TA-DAAAA!!!! (and More)

After only a couple of days, colonies will start to grow and grow and grow. It's fun to watch the progress of a colony take off, and you can get some phenomenal colors and structures. In any given swab, you may get bacteria, fungi, or molds (a type of fungi), all of which have an amazingly diverse set of characteristics. They might be round, irregular, fuzzy, bright, or dull. Here's a great starting guide to interpret what you've found!

This is from five days after, and it looks like our toilet seat had some mold spores that are now growing like crazy in the to left quadrant. As for dog mouth vs. human mouth, I'll call it a tie! And the kitchen counter looks all right!

If you look really close at your colonies, you can see how they grew and the amazing shapes that they take as they interact with one another. It's a great thing to sketch and photograph over time, as your mini-farm empire grows. Interesting things to follow up with are the FDA's factors for microbial growth, this starter on bacteria multiplication, and looking at the history of understanding microorganisms.

Try this with classes! Try changing the recipe, the swabbing, the time, the temperature, everything! Your world is a science experiment waiting to happen. Have fun, and keep exploring.

Let me what you grow!