Ask A Scientist Molecular Biology Archive

name Ken
status other
grade 9-12
location WA

Question - Why are petir dishes inverted for incubation?
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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.
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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 
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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.
 
vanhoeck
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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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