Inverted Petri Dishes
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.
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
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
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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Update: June 2012