Wikipedia
The 3M Petrifilm plate is an all-in-one plating system made by the Food Safety Division of the 3M Corporation. They are heavily used in many microbiology-related industries and fields to culture various micro-organisms and are meant to be a more efficient method for detection and enumeration compared to conventional plating techniques. A majority of its use is for the testing of foodstuffs.
Petrifilm plates are designed to be as accurate as conventional plating methods. Ingredients usually vary from plate to plate depending on what micro-organism is being cultured, but generally a Petrifilm comprises a cold-water-soluble gelling agent, nutrients, and indicators for activity and enumeration.
A typical Petrifilm plate has a 10 cm(H) × 7.5 cm(W) bottom film which contains a foam barrier accommodating the plating surface, the plating surface itself (a circular area of about 20 cm), and a top film which encloses the sample within the Petrifilm. A 1 cm × 1 cm yellow grid is printed on the back of the plate to assist enumeration. A plastic “spreader” is also used to spread the inoculum evenly.