|A Petri dish culture of a Legionella sp. The genus includes Legionella pneumophila which causes Legionnaires' disease. This is a streak plate, a method used to dilute down a sample to the point where individual colonies can be seen and then counted.|
Bacteria, fungi, and other life cannot simply be grown in a plastic dish. They need a source of food and water. This is provided through some sort of growth medium that is added prior to the addition of the bacteria or other lifeforms. This medium is usually agar-based, agar being an extract from red algae that gels in a similar manner to gelatin. Nutrients are added to the mix, the types of nutrients depending on what is being grown on the plate. Trypticase soy agar, for example, is a very basic medium that a lot of bacteria will readily grow on. After the addition of the growth medium, Petri dishes can referred to as Petri plates.
There are a variety of specialized nutrient media that are used when more information is needed beyond a basic "yep, it's a bacterium." There are selective media, which will prevent the growth of certain lifeforms. This can be helpful when, for example, a scientists wants to know what bacteria are present in a soil sample. Fungi in the soil can overwhelm a plate unless the sample is grown on a medium that inhibits fungal growth.
|A plate showing Alpha, Beta, and Gamma hemolysis|
One of the most important parts of the process of growing something in a Petri plate is cleanliness. Aseptic technique is vital to prevent contamination of samples, which would skew results. This includes everything from flame sterilization to autoclaving materials; anything that can possibly be done to keep unwanted contaminants away from the sample. I'll never forget the hours I spent in the laboratory during my days learning Microbiology, going through the procedures for aseptic technique over and over again.
Interestingly enough, after an inoculum is added dishes are actually incubated upside down, which may surprise many of you. One of the main reasons for this is to prevent contamination, as spores from various living things then cannot fall down onto the growth medium. Also, condensation cannot drip down from the lid, which would interfere with the otherwise rather stable conditions the Petri dishes provide.
Source is Microbiology with Diseases by Taxonomy, 2nd edition. Images are from Wikimedia Commons and are under Creative Commons licensing or are copyright free: one, two.