<P> Modern Petri dishes usually feature rings and / or slots on their lids and bases so that when stacked, they are less prone to sliding off one another . Multiple dishes can also be incorporated into one plastic container to create a "multi-well plate". While glass Petri dishes may be reused after sterilization (via an autoclave or one hour's dry - heating in a hot - air oven at 160 ° C, for example), plastic Petri dishes are often disposed of after experiments where cultures might contaminate each other . </P> <P> 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 or antibiotics . Once the agar cools and solidifies, the dish is ready to be inoculated ("plated") with a microbe - laden sample . Virus or phage cultures require a two - stage inoculation: after the agar preparation, bacteria are grown in the dish to provide hosts for the viral inoculum . </P> <P> Petri plates are incubated upside - down to lessen the risk of contamination from airborne particles settling on them and to prevent the accumulation of any water condensation that may otherwise disturb or compromise a culture . </P> <P> While Petri dishes are widespread in microbiological research, smaller dishes tend to be used for large - scale studies in which growing cells in Petri dishes can be relatively expensive and labor - intensive . </P>

Why are bacterial agar plates turn upside-down when they are placed in the incubator to grow