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Inverting Petri plates after they cool reduces the risk of contamination by air-borne particles. Inverting the petri dish makes it more difficult for contaminants to enter the growing medium.
When in the incubator growing cultures, Petri dishes are inverted because moisture condenses on the lid and will drop down onto the agar/growth medium causing moisture problems. If there are separate bacteria colonies on the growth medium, dripping water droplets would cause the colonies to be able to migrate and mix with each other.
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