Ever wanted to grow bacteria for a scientific project or just for fun? It turns out to be super easy – all you need is a nutrient agar (a special growth ingredient like gelatin), a few sterile petri dishes, and some disgusting sources of bacteria!
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Method 1 of 3: Preparing the Petri dish
Step 1. Prepare the agar
Agar is a jelly-like substance used to breed bacteria. This agar is made from a type of red algae that provides a growing medium surface for various types of bacteria. Some types of agar contain additional nutrients (such as sheep's blood) that help promote faster bacterial growth.
- The easiest type of agar to use for this experiment is nutrient agar which is in powder form. You'll need 1.2 grams (about half a teaspoon) of agar powder for each 4-inch (10 cm) petri dish you want to use.
- In a heatproof saucer or bowl, mix half a teaspoon of powdered nutrient agar with 60 ml (about 1/4 cup) of hot water. However, multiply this quantity by the number of petri dishes you want to use.
- Place the bowl or saucer in the microwave and simmer for a minute, watching to make sure that the solution doesn't overflow.
- When the solution is ready, the agar powder should have completely dissolved and the liquid should be clear.
- Allow the agar solution to cool for a few minutes before continuing.
Step 2. Prepare the petri dish
Petri dishes are small flat-bottomed containers made of glass or clear plastic. Petri dishes have two parts – top and bottom – which are connected to each other. This protects the contents of the cup from unwanted contaminated air, while also removing any gases produced by bacteria.
- Petri dishes must be thoroughly sterilized before being used to grow bacteria, otherwise the results of the experiment may be affected. Newly purchased petri dishes should be pre-sterilized and covered in a plastic container.
- Remove the petri dish from the container and open the two halves. Very carefully, pour the warm agar solution over the bottom of the petri dish – just enough to form a layer on the bottom of the dish.
- Quickly cover the top of the petri dish to prevent any airborne bacteria from contaminating the experiment. Keep the petri dish for 30 minutes to 2 hours, until the agar solution has cooled and hardened (when ready, the agar solution will be similar to Jell-O).
Step 3. Put the petri dish in the refrigerator until ready to use
If you don't plan on using the agar-filled petri dishes right away, they should be refrigerated until you're ready to continue the experiment.
- Storing the petri dish in the refrigerator will prevent the water in the dish from evaporating (bacteria need a moist environment to grow). It also allows the surface of the agar to harden slightly, which prevents it from tearing as you move your bacterial sample.
- When storing petri dishes in the refrigerator, they should be placed upside down. This prevents condensation on the lid that might fall to the bottom and interfere with surface growth.
- Petri dishes filled with agar can be stored in the refrigerator for several months. When you are ready to use it, remove the cup from the refrigerator and allow it to come to room temperature before adding your sample.
Method 2 of 3: Growing Bacteria
Step 1. Put the bacteria in a petri dish
Once the agar solution has hardened and the petri dish is at room temperature, you're ready for the fun part – introducing the bacteria. There are several ways to do this – through direct contact or through sample collection.
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Direct contact:
This is done when bacteria are transferred to a petri dish using direct contact, for example touching agar. One of the most frequently used ways to make direct contact is to simply press your fingertips (either before or after washing your hands) gently against the agar surface. However, you can also try pressing your fingernail or the surface of an old coin against the agar, or even putting a thin strand of hair or a drop of milk into the saucer. Use your imagination!
- Sample collection: This way, you can collect bacteria from almost any surface and transfer them to a petri dish, all you need is a few clean cotton swabs. Just take a cotton swab and wipe it over any surface you can think of – inside your mouth, doorknobs, the buttons on your computer keyboard or the buttons on your remote – then, wipe it over the agar surface (without tearing it). These spots contain a lot of bacteria, and should produce interesting (and disgusting) results within a few days.
- If you like, you can put more than one sample of bacteria in each petri dish – all you have to do is divide the dish into four sections (quarters) and wipe a different sample of bacteria on each side.
Step 2. Give a name and close the petri dish
Once you have introduced the bacteria, you must close the lid of the petri dish and isolate it.
- Be sure to name each petri dish with the source of the bacteria, otherwise you won't know which bacteria it came from. You can do this with tape and markers.
- As an added precaution, you can place each petri dish in a sterile plastic bag. This will provide additional protection from any harmful bacterial colonies that might develop, but will still allow you to see the contents of the petri dish.
Step 3. Place the petri dish in a warm and dark place
Leave the petri dish in a warm, dark place, where bacteria can thrive, undisturbed, for several days. Remember to store it upside down, so that bacterial growth remains undisturbed by any water droplets.
- The ideal temperature for growing bacteria is between 70 and 98 degrees F (20-37 degrees C). If needed, you can place the petri dish in a cooler place, but bacteria will grow much more slowly.
- Allow the bacteria to grow for 4-6 days, as this will allow sufficient time for the culture to grow. Once the bacteria start to grow, you may notice an odor coming from the petri dish.
Step 4. Record your results
After a few days, you will see different types of bacteria, fungi, and fungi growing in each petri dish.
- Use a notebook to record your observations of the contents of each petri dish and perhaps infer the places with the most bacteria.
- Is it in your mouth? Door handle? The buttons on your remote? The results may surprise you!
- If you like, you can measure the daily growth of the bacterial colonies by using a marker to trace a circle around each colony at the bottom of the petri dish. After a few days, you should have a collection of concentric circles at the bottom of the petri dish.
Step 5. Test the effectiveness of the antibacterial agent
An interesting variation in this experiment is to put an antibacterial agent (hand sanitizer, soap, etc.) into a petri dish to test its effectiveness.
- After you have placed the bacteria in the petri dish, use a cotton swab to place a small drop of hand sanitizer gel, disinfectant soap, or household bleach onto the bacterial sample center, then continue the experiment as usual.
- As the bacteria in the dish grows, you will see a ring or "halo" around where you put the antibacterial agent where no bacteria are growing. This place is known as the "clear zone" (or more accurately the "inhibition zone").
- You can measure the effectiveness of different antibacterial agents by comparing the size of the clear zone in each petri dish. The wider the clear zone, the more effective the antibacterial agent.
Method 3 of 3: Get Rid of Bacteria Safely
Step 1. Take proper precautions
Before you attempt to dispose of your petri dishes, you first need to take the proper precautions.
- While most of the bacteria you grow are harmless, large colonies of bacteria can pose a greater risk – so you need to kill them first before disposing of them with household bleach.
- Protect your hands from bleach by wearing rubber gloves, protect your eyes with plastic laboratory goggles, and protect your clothes by wearing an apron.
Step 2. Pour the bleach into a petri dish
Open the petri dish and carefully pour a small amount of bleach over the bacterial colony, holding the dish over the sink. This will destroy the bacteria.
- Be careful not to let the bleach touch your skin, as it will burn your skin.
- Then, put the disinfected petri dish back into the sterile plastic and throw it in the trash.
