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 culture.
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.
As compared to some of the other alternatives like gelatin, agar has a number of advantages which include:
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 experiments.
** Note- students should always be supervised when handling agar
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.
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.
For 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.