Petri dishes are used in microbiology labs to hold culture media where cells are cultured under controlled conditions and are examined.
Petri dishes play a vital role in the maintenance of cleanrooms, where a settled plate method is used to identify the number of colonies in a cleanroom after incubation in the micro lab.
When you observe a petri dish it is a small dish having a transparent body made of glass and a transparent lid made of glass.The culture media is placed in the body of the petri dish and is closed with the glass lid.
A recommended practice for the majority of the cases is that these plates are incubated in the incubator in an inverted position, meaning upside down, where the body of the petri dish comes up with a lid on the downward side.
Did you ever think about why Petri dishes are placed in an upside-down position?
There may be the following possible reasons
- Prevent Condensation
- Media Separation
- Prevent Media Dryness
- Prevent Contamination
- Prevent Mix Up
Prevent Condensation
The culture media in petri dishes consists mainly of water, and these petri dishes are incubated in an inverted position after solidification of the media. If the petri dish is incubated in a normal position with having lid on upside then the water will evaporate and water droplets condense on the lid of the petri dish.
These condensed drops of water will fall down in the media and will result in a colony mix up creating a difficulty in colony identification and counting.
Media Separation
The media adheres to the petri dish, and if it is incubated in a normal position due to the accumulation of water in the Petri dish (from water condensates), the media may be separated from the petri dish and will slide.
Prevent Media Dryness
The loss of water from the petri dish through evaporation is high when these are placed in a normal position, resulting in the dryness of the media.
An optimal level of moisture is required for microbial growth, so incubating the Petri dishes in an inverted position slows down the water evaporation and keeps the media hydrated.
Prevent Contamination
When the water condensates fall back on the media they may contaminate the media with contamination if present on the lid.
As the lid is larger than the body of the petri dishes, it may be mistakenly removed during handling & will result in contamination.
Prevent Mix Up
Petri dish lids are not fixed, so they may be exchanged when placed in normal conditions. So we incubate in an upside-down position and place an identification label on the body part to avoid a mix-up.
Gas Exchange
The inverted position of the petri dish allows better gas exchange between the environment and the media, which is required for microorganisms.
Conclusion
A standard practice for incubation of petri dishes in the microbiological labs is to incubate the petri dishes in an inverted position, with the exception of a few cases, which are mentioned in the standards. Must ensure the solidification of media before inverting.
The upside-down practice helps to prevent media dryness, ensure easy identification and prevent contamination.
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