Why Are Agar Plates Turned Upside Down When Incubated?

Before they are inoculated and incubated, agar plates are turned upside down so that any condensation that might be present on the lid does not drip down into the culture medium. The agar plate lid is not placed on a counter top or elsewhere, and its interior not touched, so that no airborne contaminants can affect the culture. When placed into the incubator, the agar plate remains upside down so that the culture continues to be protected from possible contamination.

Agar plates are Petri dishes that contain a medium suitable for culturing microorganisms. They may also be called blood agar plates when they contain red blood cells, often from a horse or sheep, which are often used as a nutrient. Because some bacteria can only digest red blood cells that have already been broken down, some agar plates contain a preheated medium, which breaks down the blood cells and gives them a brownish color. This is often referred to as “chocolate agar” because of its color, and it does not reflect the presence of chocolate as a growth medium.

Agar plates may also contain antibiotics that will prohibit the growth of certain strains of bacteria. This serves to make the plate selective for identifying specific kinds of bacteria by eliminating the possibility of growth by other strains. These are called “restrictive” or “selective” plates. Those agar plates that will allow the growth of whatever organisms they are inoculated with are referred to as “permissive” plates.