Agar is a jelly-like substance that creates a nice surface for bacteria, algae or fungi to grow on. These tiny animals all like to live in different environments. Algae live in the sea and like a salty environment. Moulds (fungi) like sugars. Therefore, depending on which nutrients you add to the agar, you can select for certain microbes or even single species to grow in the agar plates!
To make agar plates you use Petri dishes. Petri is actually a person! A long time ago mister Petri was investigating bacteria using gelatine. But the gelatine was too jelly and he did not succeed in getting a nice flat layer of bacteria. His wife was using agar for cooking and suggested to try that. It worked much better and Petri was able to do a lot more research on bacteria. Now the dished are called Petri dishes!
Below you’ll find a recipe to make nutrient agar plates. Nutrient agar is mix of nutrients that most bacteria like. In using this, you can grow and visualise a wide variety of bacteria.
This is what you need:
- Heating plate
- Pressure cooker
- Glass bottle (heat resistant, that fits the pressure cooker in a standing way)
- Petri dishes (for 1L 60 – 80)
- Digital scale
- 1L (tap)water (there should be no chlorine in the water!)
- 3g Bouillon
- 5g Peptone or tryptone
- 12g Agar Agar (you can buy this in Chinese shops)
1. Calculate how much you need of all the ingredients. Note: you can only use half the volume of the glass bottle.
2. Weight all ingredients. You can use a paper to do that. If you don’t have a very precise scale round off on whole numbers.
3. Pour the ingredients into the bottle and add the water.
4. Screw the lid and shake well until the big parts are dissolved.
5. Now unscrew the lid half way. This is important. Otherwise your bottle will explode!
6. Place the flask into the pressure cooker. Note: The layer of water in the pressure cooker must be 1cm lower than the water level in the flaks.
7. Sterilise the nutrient agar. To do this the pressure cooker must be under pressure for at least 20 minutes:
Turn on the stove or heating plate. Start counting the 20 minutes when the weight in the lid is upwards.
Now you can lower fire. (to save energy). But make sure the pan stays pressurised.
8. After the sterilisation, let the nutrient agar cool down until you can hold the bottle in your bare hands.
9. In the meantime you can label the Petri dishes on the bottom with name, date and content.
10. When you can hold the bottle, light the flame.
11. Close to the flame, pour 0.5cm of agar in each dish. Let the nutrient agar cool down.
12. When the nutrient agar has solidified, store the plates upside down in the fridge.
Find instructions on how to grow bacteria in your DIY nutrient agar in these DIY bacteria detector instructions. Have fun!