Petri Dishes: What are They & Other Common Questions

NICOLE ENAD     30th Jan 2025

Petri Dishes: What are They & Other Common Questions

A Petri dish, also called a Petri plate or cell-culture dish, is a simple, flat, round container with a lid. It's made of clear glass or plastic and is used by scientists to grow cells, bacteria, fungi, or even small plants. These dishes are named after their inventor, German scientist Julius Richard Petri, who first used them to culture bacteria back in the late 1800s.

Over the years, Petri dishes have become an essential tool in laboratories around the world. They've been used to discovering many important things, like how bacteria grow, how diseases spread, and how new medicines, like antibiotics, work.

These humble dishes have been at the heart of many important discoveries in medicine and microbiology. For example, in the 1920s, researchers used Petri dishes to study the scarlet fever bacteria and show why it's important to keep milk refrigerated. They have also been used to test the effectiveness of antibiotics or to grow different types of bacteria under various conditions. The use of Petri dishes even helped to identify the bacteria responsible for outbreaks like the plague, thanks to scientists like Alexandre Yersin.

Perhaps the most famous story linked to Petri dishes is the discovery of penicillin. In 1928, Alexander Fleming, while sorting through his Petri dishes, noticed something strange: a clear zone around a blob of mould. This observation led to the discovery that the mould, later identified as Penicillium notatum, could kill bacteria, ultimately leading to the creation of penicillin – a breakthrough in medicine that saved millions of lives.

Since its invention, Petri dishes have become one of the most important laboratory equipment. In this blog, we’ll find out in detail how to use Petri dishesin science equipment laboratories & its various purposes.

What are Petri dishes?

A Petri dish is a shallow, round glass container used in laboratories. It is commonly used to grow microorganisms like bacteria, viruses, and fungi.

The petri dish provides a controlled environment for these tiny organisms to grow and be studied. To grow microorganisms, a special substance called a culture medium is added to the dish. This medium contains nutrients that support the growth of microorganisms.

The Petri dish keeps the microorganisms isolated from the outside environment, allowing scientists to study them closely and carefully.


                                     

Types of Petri Dish

Based on shape

Usually, Petri dishes are cylindrical, with diameters ranging from 30 to 200 millimeters. Square and rectangular-shaped Petri dishes are also available.

Based on usability

They may be reusable or disposable, depending on the type of materials they are made of. Reusables are durable in the sense that they can withstand repeated dry/wet sterilization procedures.


Based on the type of material

They can be made of glass, polystyrene, polypropylene, or cellulose acetate.

  • Glass: Borosilicate glass is often used, which is heat resistant and suited for sterilization in autoclaves or even in laboratory ovens at 120-160°C.
  • Plastic: Plastic petri plates are a good choice if cross-contamination is a problem, as they are disposable. However, they have poor low-temperature performance.
  • Polystyrene Petri dishes are disposable and cheap with their wider performance in industrial and clinical settings for routine microbiological testing.
  • Polypropylene petri dishes are lightweight, heat and chemical resistant, and more durable than glass Petri dishes
  • Cellulose acetate: This type of petri dish is particularly relevant where there is a substantial requirement of oxygen and carbon dioxide, thus facilitating greater gas permeability.
  • Nitrile petri plates are a good choice for low-temperature applications with a temperature range of -30 F to 275 F.


Based on compartments

Compartments in Petri dishes allow the preparation of multiple culture media in a single plate. The number of compartments may range from 2 to 4.

Why Use Petri dishes in a laboratory?

A Petri dish is a key tool in biology and chemistry labs. It is used to grow cells and keep them safe from contamination. The dish is clear, so it’s easy to see the growth of microorganisms. Its size allows it to be placed directly under a microscope for observation, without needing to move the contents to another plate. In schools and colleges, Petri dishes are often used to watch how seeds grow.

Uses of Petri dishes

  • It is used to culture cells in the storage space present in it. One of the reasons to use this method is also to protect cells from contamination. Also, the transparency of glass/plastic helps to observe the growth of organisms without disturbing them.
  • Sometimes, it can also be used in schools and colleges for science projects like the observation of seed germination.
  • Some petri dishes can be used to measure the density of culture with the help of grids printed on their bottom.
  • They are used for the storage of samples in liquid or granular form or small objects like insects and seeds.
  • They are also used in the evaporation and drying of samples.
  • One of the common applications of Petri plates is to make agar plates. Here is the simple procedure for making an agar plate:


How to use Petri dishes effectively in a laboratory

Before using a Petri dish, make sure it is clean and free from any particles that could affect the experiment. To clean it, treat the dish with bleach and sterilize it for reuse. Always sterilize the dish before using it.

To observe bacteria growth, first fill the dish with agar medium. This medium, made with red algae, has nutrients, blood, salt, indicators, and antibiotics that help microorganisms grow. Store the Petri dishes in the refrigerator upside down. When you need the culture plates, remove them from the refrigerator and use them once they are back at room temperature.

Next, take a sample of bacteria or another microorganism. Slowly pour it onto the culture or use a cotton swab to apply it in a zigzag motion. Be gentle to avoid damaging the culture.

Afterward, cover the dish with a lid. Store it at around 37ºC for a few days to let it grow. After a few days, your sample will be ready for further study.

Aftercare:

  • After using the dish, treat it with bleach and sterilize it.
  • Sterilize the dishes even before using them.
  • Store the petri dish in the refrigerator or any storage system upside-down.

Precautions:

  • Sterilization or disposal of Petri dishes should be done before and after their use.
  • Gloves should be worn while using Petri dishes.
  • Experiments should be performed in a sterile environment.


Why is the petri dish placed upside down during incubation?

The petri dish is placed upside down during incubation to stop water from evaporating onto the lid.

When the dish is in a normal position, water can evaporate from the agar and collect on the lid. This water can drip down onto the bacteria, mixing them up and causing uneven growth. This makes it harder to count the bacteria correctly.

When the petri dish is upside down, less water evaporates from the agar (the jelly-like substance where bacteria grow). This helps keep the agar moist instead of letting it dry out. Moist agar is important because it gives the bacteria a better environment to grow in. If the agar dries out, the bacteria might not grow properly.

Also, by flipping the dish, any dust or germs (called contaminants) are more likely to land on the lid instead of the bacteria. This reduces the risk of contamination and keeps the bacteria safe.

It’s also easier to handle the dish when it’s upside down.


Advantages of Petri Dish

  • It provides the storage space for culturing the cells.
  • It also helps to avoid contamination.
  • It also comes in different sizes and shapes, which broadens its application.
  • The transparent feature of the petri dish helps observe cells inside it without needing to remove the lid.
  • Reusability after sterilization is another advantage of using a glass petri dish.


Limitations of Petri Dish

  • One of the limitations of using a Petri plate is the need for skilled users. Otherwise, contamination is always a possibility.
  • The material used to make Petri dishes is another limitation. Plastic and glass might not be biocompatible or affect experiments dealing with sensitive cells.
  • Likewise, it doesn’t mimic the physiological conditions where the cells/microbes grow.


Where to Buy Petri Dishes:

If you're looking to buy Petri dishes for your laboratory or research needs, MedCart is the perfect place to shop! As Australia's #1 medical marketplace, we offer a wide range of high-quality Petri dishes for all your scientific requirements.

Visit our online store today and explore our collection of Petri dishes: MedCart - Petri Dishes.

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