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storing agar plates - why upside down? (Apr/24/2009 )

Hallo all,

why are agar plates stored upside down?

I have read somewhere its because of the condese? to prevent the agar to get too wet?

But why should I put the plates still upside down after I placed my organism on it? (why do you put them upside down in the incubator?)



In the fridge, because of the condensation.
In the incubator, once the petri dish is dried, there is no condensation, I don't know. Maybe because I'm used to keep it upside down, maybe also because their could be less contamination because dust could not fall on the agar... There is no reason not to put it upside down.

-little mouse-

Its because gas is produced by the dividing bacteria which if the plates are kept agar side down will rise to the lid, condense on it it as it cools and then fall back on the colonies as liquid and thus they will smear your colonies.

When the plates are kept agar side up the condensation falls on the lid and evaporates without touching the bacteria


I agree with cotchy -- that's why I do it. Also, it's good practice to label the bottoms of the plates rather than the lids, 'cause lids can be accidentally switched. So, lid side down is also label side up.


Thanks a lot of the answers.


The Real Reason ;-) is that it lets us differentiate the initiated from the uninitiated. Who, but One Who Knew would put them into an incubator upside down.

But it does keep condensation from falling on the colonies, too.