Petri dishes are often used to make agar plates for microbiology studies. The dish is partially filled with warm liquid containing agar and a mixture of specific ingredients that may include nutrients, blood, salts, carbohydrates, dyes, indicators, amino acids and antibiotics. After the agar cools and solidifies, the dish is ready to receive a microbe laden sample in a process known as inoculation or "plating." For virus or phage cultures, a two-step inoculation is needed: bacteria are grown first to provide hosts for the viral inoculum.
Petri plates are incubated upside down to lessen the risk of contamination from settling airborne particles and to prevent water condensation from accumulating and disturbing the cultured microbes. Scientists have long been growing cells in natural and synthetic matrix environments to elicit phenotypes that are not expressed on conventionally rigid substrates. Unfortunately, growing cells either on or within soft matrices can be an expensive, labor intensive, and impractical undertaking.