Do you want to do a bacterial culture for a science project or just for fun? It's surprisingly simple, all you need is agar (a nourishing gelatinous substrate), sterilized Petri dishes and some disgusting sources of bacteria!
Steps
Method 1 of 3: Part 1: Prepare the Petri dishes
Step 1. Prepare the agar
Agar is the gelatinous substance used in bacterial cultures. It is made from red algae and offers an ideal surface for the growth of many types of bacteria. Some types of agar contain added nutrients, such as sheep blood, which help the growth of bacteria.
- The simplest type of agar to use is powdered agar. You need 1.2 grams (about half a teaspoon) of powdered agar for every 4 Petri dishes.
- Using a heat resistant cup, mix the half teaspoon of agar with 60ml of hot water. Multiply these quantities according to the Petri dishes you intend to use.
- Put the cup in the microwave and boil for 1 minute, checking that the mixture does not overflow.
- When the solution is ready, the agar powder will have dissolved completely and the liquid will have a light color.
- Let it cool for several minutes before proceeding.
Step 2. Prepare the Petri dishes
These dishes are small flat-bottomed containers of glass or clear plastic. They have two halves, upper and lower, which fit into each other. This serves to protect the contents from unwanted contamination, letting out the gases produced by the bacteria.
- The dishes must be completely sterilized before they are used to grow bacteria, otherwise the result will be affected. New dishes should be sold in sealed and sterilized plastic sleeves.
- Take the plate out of its packaging and separate the two halves. Carefully pour the warm agar into the bottom half of the Petri dish, just enough to form a layer on the bottom of the dish.
- Close quickly with the lid to avoid any contamination by bacteria present in the air. Set the dishes aside for 30 minutes to 2 hours, until the agar has cooled and solidified (it will eventually look like jelly).
Step 3. Refrigerate the dishes until use
If you are not going to use them immediately, you should put them in the fridge until you are ready to start the experiment.
- Storing in the fridge prevents the evaporation of water inside the dish (bacteria need a humid environment to grow). It also allows the agar surface to harden slightly, preventing cracks or scratches when transferring the bacteria samples.
- Place the dishes in the fridge upside down. This will prevent condensation on the lid from falling onto the growing surface and damaging it.
- Dishes filled with agar will keep in the fridge for a couple of months. When you are ready to use them, take them out of the fridge and let them reach room temperature before using them.
Method 2 of 3: Part 2: Growing the bacteria
Step 1. Introduce the bacteria to the Petri dish
Once the agar has solidified and the dish is at room temperature, you can move on to the fun part, which is introducing the bacteria. There are two ways to do this: through direct contact or by transferring a sample.
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Direct contact:
It occurs when bacteria are transferred to the agar by direct contact, for example by touching it. One of the most common methods of doing this is to press your fingers (before or after washing your hands) lightly on the surface of the agar. You can also touch the surface with a fingernail or a coin, or by placing a hair or a drop of milk on the plate. Use your imagination!
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Collecting a sample:
With this method you can take bacteria from any surface and transfer them to the Petri dish. All you need is a few cotton swabs. Pass it on the chosen surface (the inside of the mouth, a handle, the keys, the computer keyboard, the remote control keys), then rub it on the surface of the agar, without breaking it. These surfaces contain a lot of bacteria, and they should give interesting (and disgusting) results within a couple of days.
- If you prefer, you can put more than one sample in each plate, you just have to divide the area into quadrants (quarters) and transfer a different sample into each one.
Step 2. Label and seal the Petri dish
Once the bacteria have been introduced, you need to close the dish and seal it with tape.
- Make sure you label each dish with the source of the bacteria it contains, or you won't be able to tell them apart anymore. You can do this with tape and a marker.
- As an extra precaution, you can put the plate in a zip-up bag, which gives additional protection against any danger of contamination by dangerous bacteria that could develop, while giving you the opportunity to look inside the plate.
Step 3. Place the Petri dishes in a warm and dark environment
Store the dishes in a warm, dark environment where bacteria can grow undisturbed for several days. Remember to store them upside down, so that the bacteria are not disturbed by the condensation drops.
- The ideal temperature for the growth of bacteria is between 20 and 37 degrees. If necessary, you can grow them at a lower temperature, but they will grow much slower.
- Let the crop develop for 4-6 days. As it grows, you will notice an odor coming from the dishes.
Step 4. Note the results
After a couple of days you will notice a stupendous variety of bacteria, molds and fungi growing inside the petri dishes.
- Record your observations on the contents of each dish on your computer and try to figure out which surface contained the most bacteria. Is it the inside of your mouth? The door handle? The buttons on the remote control? The result may surprise you!
- If you want you can measure the daily growth of the bacteria colonies by using a marker to circle the colony at the bottom of the dish. After several days you should have a series of concentric circles on the bottom of each plate.
Step 5. Test the effectiveness of antibacterial agents
An interesting variation of this experiment is to introduce an antibacterial agent (such as hand soap) into the dish, to test its effectiveness.
- Once the bacteria are on the dish, use a cotton swab to introduce a drop of liquid soap, disinfectant, or bleach into the culture medium, and let the experiment continue.
- Over time you should see a halo where you put the antibacterial agent, where bacteria shouldn't grow. This is called the "dead zone".
- You can measure the effectiveness of different antibacterial agents by comparing the size of the dead spots on each plate. The wider the area, the greater the effectiveness of the antibacterial agent.
Method 3 of 3: Part 3: Safely dispose of bacteria
Step 1. Take necessary precautions
Before throwing away the dishes you need to take the necessary safety measures.
- While most of the bacteria you grow aren't dangerous, larger colonies can pose a risk, so it's best to destroy them before throwing them out using bleach.
- Protect your hands from bleach by using rubber gloves, wear goggles, and put on an apron.
Step 2. Pour the bleach into the Petri dishes
Open the dishes and carefully pour a small amount of bleach over the bacterial colony, working on a sink. This will destroy the bacteria.
- Be very careful not to come in contact with the bleach as it burns.
- Then put the disinfected dish in a zip-up bag and throw it in the trash.
