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How to Make Agar Petri Dishes to Grow Bacteria

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Learning about the world around us includes the microscopic world and that is why I am sharing how to make agar petri dishes to grow bacteria and explore the tiny environments in our home!

Disclosure: I am partnering with Bradley Corp. All opinions are my own.

How to make agar petri dishes

How to Make Agar Petri Dishes to Grow Bacteria

My kids are always asking me to do experiments with them. A lot of at-home experiments I have to be cautious with because in my specialty (microbiology) that can be hazardous, but recently I learned how to make agar petri dishes at home and wanted to see how it worked to grow bacteria.

Ingredients to make Agar

Agar is a growth medium that bacteria can grow and reproduce on. It is often found in petri dishes and test tubes and usually comes in a powder that you prepare. However, you can also create your own growth medium at home, using supplies that are found in your kitchen! Grab the ingredients and follow along to see how to make agar petri dishes to grow bacteria at home.


  • 1 tsp Unflavored Gelatin
  • 1 tsp Sugar
  • 1 cube Beef Bouillon
  • 2 cups Water

Step 1: Mix All Ingredients together

Before you start, dissolve the beef bouillon in 2 cups of hot water. Once it is dissolved, use JUST 2 tsp of the beef bouillon broth in the agar mixture. You can store the rest of the beef bouillon in the fridge for whatever soup or delicious meal your heart desires. Then go ahead and mix the gelatin, water, sugar and bouillon solution together in a saucepan.

How to make agar petri dishes

Step 2: Heat until boiling

Heat the mixture up to boiling, stirring occasionally to ensure that everything fully dissolves. Once it begins to boil, remove it from the heat and let it cool down. Check the solution every 5 minutes to make sure that it is not beginning to gel up.

Step 3: Pour the Agar

Once the agar is cool BUT NOT solidifying, you can pour it into a clear container. I have access to petri dishes and you can also purchase some on Amazon if you want. I usually pour my agar solution into a measuring cup and then pour it into the petri dishes to lessen the spill factor.

Petri dishes in fridge

Step 4: Cool It Down

Make sure to cover your containers to avoid any air contaminants that naturally live in the environment. Then place your containers in the fridge overnight and allow them to cool and solidify. You can see that my agar is very brown. You actually want more of a yellow brown color. I used too much bouillon. Plates are good for about 1-2 days in the fridge.

Step 5: Swab for Microbes!

Now for the fun part! Using clean cotton swabs, gently rub them on various surfaces. I like to swab surfaces that we touch often. Try to rotate the swab on the surface you are collecting from to cover it completely. Microbes are everywhere!

swabbing petri dishes

Once you have your swab, bring it over to your petri dish and remove the lid. Using a gentle back and forth zig zag motion spread your bacteria on the plate. DO NOT poke your swab into the agar. You want to do this in 3-4 overlapping quadrants as you rotate the plate so that every section of the dish has the potential to have bacteria on it. Label each plate and make sure to use a different cotton swab for each surface you swab.

Step 6: Incubate & Wait

This next part is time consuming, but worth it in the end. First, tape the dishes closed, bacteria can be pathogenic and you want whatever you grow to stay in there and NOT get out. Next you will need to find a spot in your home that is warm, but not too hot, and is dark. Bacteria love warm, moist, dark areas. A cardboard box would work great for this. Allow the plates to sit for a few days. Check on them periodically for growth.

Petri dish with bacteria growth on it

Step 7: Wash Your Hands

Do not forget this last step. Even if the surface looks clean, make sure to wash your hands. Especially after handling the plates. According to Bradley Corporation and its Healthy Handwashing Survey, most Americans wash their hands for 21 seconds on average and although soap is far better than hand sanitizer, the length of time is important with the goal being at least 20 seconds according to the CDC.

Infographic on average length of time American wash hands.

Handwashing has been at the forefront of discussions since the pandemic began in 2020, however as we hopefully near the end, people are decreasing their regular washing. Handwashing has been proven to be a safe and effective way to combat microbes and maintain good hygiene.

infographic on how often Americans wash their hands.

Making your own agar to grow bacteria on can be a really great way for you and your family to learn about microorganisms and just be more aware of what is around you and on the everyday items you come into contact with. I personally used this as a way to remind my kids about hygiene and handwashing.

Are you ready to make agar petri dishes and grow bacteria?

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Thursday 9th of March 2023

How many plates does this recipe make?


Thursday 9th of March 2023

I made 6 and could have stretched that and made thinner layers.

Mrs. Pugh

Sunday 5th of March 2023

Can you use broth instead of bullion?


Thursday 9th of March 2023

I am not entirely sure on that. You could try, but it may be soupy.


Monday 27th of February 2023

You say 3 cups of water in list of materials but say to use 2 cups of water in the actual experiment. Which one is correct?


Tuesday 28th of February 2023

2 cups for sure! I need to adjust that and I forgot!


Thursday 11th of August 2022

My plates became liquidy when they reached room temperature and had been out for a day. What can I do to avoid this?


Thursday 11th of August 2022

Hi! They actually might have gotten too warm. Also, some bacterial strains actually consume the agar and break down the gelatin. You can purchase agar powder on amazon if this continues to happen.

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