Everything You Need To Know About Petri Dish With Agar – Preparation, Requirements, and Procedure

A Petri dish, otherwise known as a Petri plate, is a shallow, glass-lidded, cylindrical dish that is generally utilized to culture microorganisms.

Everything You Need To Know About Petri Dish With Agar - Preparation, Requirements, and Procedure

Petri dishes can come in both plastic and glass forms and can be sterilized and cleaned by using an autoclave; to be used again. Before using a petri dish for culture purposes, it is important to make sure that the dish is not only clean but also; sterile.

This helps to prevent any contamination and makes for a more accurate new culture. 

In addition to this, Agar is a polymer that constitutes a variety of galactose sub-unites, as well as numerous species of red algae.

While agar has many other uses including dentistry and culinary, it plays an important part in microbiology as a culture medium for different microorganisms, chiefly; bacteria. 

When compared to other Petri dish mediums, such as gelatin, agar has a variety of different advantages such as it is firmer than gelatin, stronger, and can’t be easily degraded or eaten by microorganisms. 

Depending on the particular strain or type of bacteria being produced, there are different agar for different purposes. Although, it is important that great care and accurate handling is taken when experimenting with any culture.

For home experiments with younger students, plain nutrient agar, rather than an agar formula supporting pathogenic bacteria, is recommended. 

With this in mind, this article will inform you on everything you need to know about Petri dishes with agar, as well as how to prepare one, the requirements, and the procedure. 

What Types Of Agar Are There?

There are many different types of agar available to use in Petri dishes – each containing its own traits. While some are suitable for student use, others should only be used by professional scientists due to risks of contamination. The different types of agar include:

Blood Agar

As its name suggests, this is acquired from animal blood (typically sheep). As a result, blood agar contains animal blood cells. This can be utilized to grow most bacteria. Blood agar is not suitable for student use. 

Chocolate Agar

This is, again, acquired through sheep’s blood. It is generally used to grow Haemophilus as it contains the necessary factors (X and V) to achieve this.

This is a nutrient culturing medium and can be used to grow Haemophilus and Neisseria. The only downside is that no hemolysis data can be extracted from this agar.

If you’re looking to differentiate species of Haemophilus then you would need to perform other tests. Chocolate agar isn’t suitable for student use. 

Luria Bertani Agar (LB Agar)

This is actually a subtype of agar. It is typically used for routine cultivation and is generally used within microbiology. It can be utilized to produce microorganisms, although not Hemophilus or Neisseria. This type of agar is suitable for student use. 

MacConkey Agar

This agar is only used for producing negative bacteria. It comes in the form of powder. There are two types of MacConkey agar: one contains added sugar lactose and the other doesn’t.

Everything You Need To Know About Petri Dish With Agar - Preparation, Requirements, and Procedure (1)

This agar can be used to grow E.coli and is easily noticeable by its red colonies. Red colonies indicate that you are using sugar to develop. This agar isn’t suitable for students.

Miller’s LB Agar

This is a variation of Luria Bertani agar (LB agar). As a result, the components are the same. The difference lies in the proportions. This agar can be suitable for students but only when the generic formula is used. 

Neomycin Agar

This agar contains an antibiotic known as neomycin. This antibiotic is typically found within different medicines and ointments including eye drops. It was discovered in 1949 by Selman Waksman and has been used ever since.

Neomycin is produced by a bacterium called Streptomyces fradiae – it isn’t man-made. The effect of a neomycin agar typically results in killing gram-negative and gram-positive bacteria.

Some people are allergic to neomycin, therefore; it is potentially toxic. Neomycin can be applied to anaerobically culture microorganisms.

However, it prevents any gram-negative staphylococcus and bacilli, although; this allows species such as Streptococcus to grow. This type of agar is not suitable for students.

Non-nutrient Agar

While non-nutrient agar can’t be utilized in culturing bacteria,  it can grow other microorganisms. This is not suitable for students.

Nutrient Agar

This has the ability to grow various types of fungi and bacteria. Although, it isn’t suitable for all bacteria. This nutrient is an amalgamation of yeast and beef broth extracts.

As a result, it can sometimes be too rich for some bacteria and too deficient for others. This is suitable for students. 

Sabouraud Agar

This contains an aminoglycoside antibiotic called gentamicin. As a result of its low pH levels, this agar kills most bacteria. This is unsuitable for students. 

Tryptic soy Agar

This agar can grow many varieties of microorganisms and can be used for colony morphology. This is suitable for students.

Xylose Lysine Deoxycholate Agar (XLD agar)

When used with stool samples, it can produce gram-negative bacilli. This is not suitable for students.

How To Prepare A Petri Dish With Agar

Before carrying out the instructions, make sure that your Petri dishes remain closed; or the lid is firmly intact until the agar is ready to be poured in. 

Requirements

  • Lab thermometer
  • Agar powder
  • Glass stir rod
  • Distilled water 
  • Boiling mixture 
  • Heat-resistant hand protection
  • Beaker/ flask 
  • Sterile Petri dish

Procedure

1. Once the recommended amount of distilled water and agar is measured, place it into a sterile, clean beaker or flask. 

2. Wearing heat-protective gear, carefully hold the beaker/flask above the flame and, using a sterile stir rod, mix the contents, and continue heating. 

3. Continue doing this for one minute. Then, once completed, remove it from the heat. 

4. Place the sterile lab thermometer into the mixture and observe the temperature until it reaches 47 degrees (45-50 degrees). 

5. Pour this melted agar into the Petri, making sure it covers the bottom (approximately a quarter), and quickly replace the lid. 

6. Place the agar safe to allow it to sit and cool at room temperature. 

7. Once set, it is ready for storage. 

Tip: When storing your agar plate, make sure to place them in an inverted position (the lid should be at the bottom). This will prevent condensation from transferring onto the agar, which may transport organisms between colonies. 

Final Thoughts

While preparing agar plates can be a fun activity to do with children, there are many ready-to-use plates available at most laboratory stores. If you do not want to go through the process of preparing agar plates, then this may be the option for you. 

Typically, this activity is safe; however, it is important to follow all safety precautions in order to prevent any injuries or accidents from taking place.

Likewise, care should be taken to prevent the Petri dishes from becoming contaminated as this could introduce unwanted microorganisms into your experiments. 

Therefore, make sure to firmly close the Petri dish before and after pouring in the melted agar. If stored correctly, then; you have a fresh plate ready to culture microorganisms.

Jennifer Dawkins

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