Agar plates are the standard solid support material for growing microorganisms. Microbial growth media contains nutrients and an energy source to fuel the microbes as they grow, and agar to keep the media in a semi-solid, gel-like state.
On solid media, a single microbe will grow and divide to produce a "colony," a spot of identical descendants. Different types of microbes produce colonies with different characteristics-shape, color, texture-which help microbiologists determine if a culture is pure, or identify the types of microbes in a mixed sample.
A number of biological supply companies sell pre-made plates, but making your own is much less expensive. With a little practice, you will find that it is very easy to make your own plates, and you will have the added flexibility of being able to customize recipes to suit your needs.
Tip: Microbiologists typically use sterile, disposable polystyrene plates, but you can also use glass Petri dishes or even baby food jars. If you use glass dishes or jars, you may find it easier to sterilize the culture media directly in them. Prepare the media as in Step 3, but instead of sterilizing it, boil it until the agar is dissolved, then pour into individual containers. Sterilize the media in the containers in an autoclave or pressure cooker.