Why should the Petri dish be inverted when cultured on the plate?
In plate culture, the Petri dish must be turned upside down, mainly for the following reasons
1. Avoidance of contamination
When the culture dish is upright, water evaporated from the culture medium will condense on the lid of the dish and may cause bacteria to fall back onto the surface of the culture medium, causing contamination and affecting culture results.
When the culture dish is turned upside down, the evaporated water condenses evenly on the surface of the culture medium, and evaporation and condensation balance each other, which does not affect the culture results. Since the air in the plate cannot flow easily, infection with foreign bacteria is reduced as much as possible and contamination of the medium and culture medium by bacteria is prevented.
2. Easy access
Since the Petri dish has a large lid and a small bottom, it is easy to lift only the lid when removing the Petri dish, exposing the culture medium to the air, causing contamination or dropping the Petri dish. The inverted plate may retain its original shape when removed, and it is not easy to separate the bottom and lid of the plate, avoiding contamination of bacteria in the environment.
3. Slowing down evaporation
In a forced convection incubator, inverting the culture dish can reduce the airflow on the surface of the medium, reduce the evaporation rate of the water, and ensure that the medium does not crack as easily and does not affect the growth of microorganisms. Inverted Petri dishes are more conducive to bacterial growth and multiplication.
4. Easy to observe
Inverting the Petri dish allows the spread of colonies to be controlled to some degree, which encourages the formation of individual colonies and makes observation and counting easier.
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