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Name: Ken
Status: Student
Grade:  9-12
Location: WA
Country: United States
Date: January 2005

Why are petir dishes inverted for incubation?

If they are incubated right side up, moisture condenses on the lid and drips down on the surface of the agar which causes individual bacterial colonies to spread and mix with each other.

Ron Baker. Ph.D.

Two reasons:

1. To minimize evaporation of water.
2. To minimize contamination from extraneous bacteria.

In either case the water or contaminant has to diffuse up and around the inverted petri dish and cover. This diffusion is a slow process in the absence of strong convection of air.

Vince Calder

Condensation tends to build up as bacteria metabolize and the warm humid atmosphere of an incubator also adds to it. This prevents drops of liquid from dripping onto the surface of the plate and causing bacterial colonies to be spread around.


Why indeed? Well, it turns out that moisture often condenses on the lid of the Petri dish. If drops of this water drip off of the lid and onto the agar surface where the colonies are growing, the water can make a mess by mixing the different bacterial colonies together.

Paul Mahoney, PhD

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