Petri Dish with Agar
Preparation, Requirements and Procedure
A Petri dish (Petri plate)
is a shallow cylindrical glass lidded dish that is typically used to culture
microorganisms (agar plates). There are glass and plastic Petri dishes, and
both can be sterilized (using an autoclave) and re-used. Before being used for
culture purposes, it is important to ensure that the Petri dish is not only
clean, but also sterile. This helps prevent the contamination of the new
Agar is a polymer made up
of various sub-units of galactose and various species of red algae. Although it
has other uses including culinary and dentistry, agar plays an important role
in microbiology as culture media for a variety of microorganisms.
to some of the other alternatives like gelatin, agar has a number of advantages
- It cannot be easily degraded by microorganisms,
- It is stronger,
- It is firmer than gelatin.
Depending on the type or
strain of bacteria being grown, there are different types of agar that may be
recommended. Although great care has to be taken when handling any culture,
plain nutrient agar, rather than an agar formulation that supports pathogenic
bacteria is recommended for younger students or in the case of home
Common Types of Agar
- Chocolate agar
- Blood agar
- Macconkey agar
- Neomycin agar
- Nutrient agar
- XLD agar
**Note- students should
always be supervised when handling agar
Preparation of Agar Plates
Before starting, ensure
that the Petri dish (dishes) is closed/has its lid on until you are ready to
pour the agar in to them.
- Lab thermometer
- Distilled water
- Glass stir rod
- Heat resistant hand protection
- Boiling mixture
- Sterile Petri dish
- Measure the
recommended amount of agar and distilled water in to a clean, sterile flask or
- Using heat
resistant hand protection, hold the beaker/flask over a flame and stir the
mixture gently using a sterile stir rod while heating
- Continue boiling
the mixture for about one minute, and then remove from heat
- Place a
sterile lab thermometer in the mixture and monitor until its temperature falls
to about 47 degrees (45- 50 degrees)
- Pour melted
agar in to the Petri dish to cover the bottom (about a quarter) and replace the
- Allow the
agar plate to cool and set (the medium will set like gelatin at room temperature)
- It is
ready for storage once it sets
During storage (in
refrigerator, but not to freeze) the agar plates should be placed in an
inverted position with the lid at the bottom. This prevents the condensation
from dripping down on to the surface of the agar, which may allow for the
movement of the organisms from one colony to another.
Although preparing agar plates
may be a fun activity, ready to use agar plates are available in various
(trusted) laboratory stores. This is mostly recommended for home use for those
who may not wish to go through the process of preparing the agar themselves.
Preparation of agar plates is an easy
and fun exercise for both the students and teachers/parents. Agar powder or
agar preparation kit will also come with instructions/procedure, which allows
for a much easier and fun time during the preparation process.
Although it may
prove easy and fun for many, it is always important to ensure that all the
safety precautions are taken to avoid possible accidents and injuries. Moreover,
enough care should be taken to avoid contaminating the plate, which would
introduce other unwanted microorganisms in to the prepared agar plate.
instance, closing the Petri dish before pouring the melted agar ensures that
the Petri dish does not get contaminated. If stored properly, the plate is
ready to be used to culture the intended microorganism.
Learn more about Cell Culture, Cell Division, Cell Differentiation and Cell Staining.
As well as Tissue Culture.
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