Agar Plates vs Petri Dishes: Are They the Same?

If you’ve been wondering about agar plates vs petri dishes, you’re not alone. Many people use the terms interchangeably when they really mean one or the other. The truth is, the two are not the same, and there is quite a big difference between them. What is it?

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What Is a Petri Dish?

A Petri dish is very simple – a shallow glass dish with a tightly fitting lid. The purpose of this dish has everything to do with its differentiation from the agar plate, however. So, understanding the history will help us understand the two materials in question.

You see, in the early 1800s, scientists began examining germs under microscope. Now that they had microscopes, they could begin to try to understand what these tiny microbes were, how they interacted with human biology, and what we could do to affect them.

Louis Pasteur, the French scientist, told us all about bacteria. He discovered yeast. And from there, the world of science was forever changed.

Quite quickly, the questions began to swirl around how best to examine these microscopic living organisms.

At first, they were simply investigated under the microscope and left exposed to the open air.

Soon, however, scientists realized that this approach would not work. You see, yeast and bacteria are quite literally everywhere. As a result, when you leave one organism out for later examination, you will find that each time you attempt to examine that organism again, it has new organisms that have joined it. These things tend to gather in the same spaces.

Then, scientists took to placing the bacteria or yeast to be examined on some nutritional medium for it to grow on a glass plate and then to covering that growth with a glass bell jar.

As you can imagine, this process was rather clunky and unwieldy. Imagine having to lift a glass bell jar every time you wanted to examine your experiment.

Furthermore, every time you lifted the glass bell jar, the bacteria or yeast would then be exposed to the elements.

It was in 1887 that a scientist named Julius Petri conceived of the Petri dish, a shallow glass dish in which a growth medium could be placed and then covered by an airtight glass lid. At the time he was working for the head of the Imperial Health Office in Berlin, Robert Koch, who is known as the father of bacteriology.

Thus, the Petri dish was born. Now, the problem of that growth medium.

What Is an Agar Plate?

The Petri dish is just the dish. The agar is the growth medium inside the petri dish.

You see, you can order a number of Petri dishes and have just the dishes arrive, which would probably be rather disappointing, as the entire purpose of Petri dishes is to examine bacteria, yeast, or other germs under a microscope. Without the growth medium, you have nowhere to actually place the bacteria, nothing to hold it in place.

And certainly nothing on which it can grow.

Enter Walther Hess, or rather, Walther Hesse’s wife. Walther Hesse was a colleague of Julius Petri, also working under Robert Koch, so he was familiar with the problem of how to store, grow, and examine bacteria.

In a discussion with his wife on the subject, Hesse expressed the problem of growth medium, and his wife, Fanny Hesse, brought up the possibility of agar.

For those unaware, agar was a common kitchen staple at the time as it helped create gelatin for all sorts of cooking and baking purposes.

Agar, or agar-agar, is a concentrated form of red seaweed that gelatinizes when boiled. It can be mixed with a wide range of nutrients, so scientists can choose which nutrient may be best to grow different bacteria.

Also read: The gel substance in a petri dish: what is it?

What Is the Difference Between a Petri Dish and an Agar plate?

So you see, the difference between a Petri dish and an agar plate is that the agar plate is the growth medium that sits inside of the Petri dish in order to grow and examine bacteria.

In most cases, you cannot have one without the other, but the two are not the same.

Also note that agar does not have to be made from red seaweed. It can also be made from gelatin, but gelatin is made from animal products, and in some cases it is easier to work with vegetarian products.

How to Make Your Own Agar or Gelatin

To get a sense of what agar was like before agar-agar came along, you can try making your own. That’s how simple it is.

To make your own agar, you will need:

  • 1 teaspoon of beef stock powder
  • 1 cup of water
  • 1 teaspoon of sugar
  • 1 teaspoon of gelatin
  • 1 Saucepan
  • 2 Petri dishes

Bring the water to a boil in the saucepan and add the beef stock, gelatin, and sugar. Stir until it is all dissolved.

Turn off the burner and allow the combination to cool for about 10 minutes.

You don’t want the gelatin to fully set yet.

Remove the lid from the Petri dishes and pour half of the mixture in each container. Immediately put the lid back on and place the dishes in the refrigerator for at least 4 hours.

The gelatin will solidify in that time and be ready to grow bacteria.

If you want the vegetarian version of this mixture, you can find a vegetarian gelatin and just use sugar instead of adding the beef stock.

Each type of agar or gelatin is suitable for different types of growth media.

Summing It All Up

In the end, the difference between Petri dishes and agar plates is important to understand in terms of usage and ordering supplies. You don’t want to find yourself with only Petri dishes and no agar, and you don’t want to order only agar with no storage.

Once you figure out you need both, you’re in good shape.


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