|MadSci Network: Microbiology|
Refigerating the nutrient agar does two things, it helps to keep it from drying out, and it helps to keep it from going bad. Cold tends to help things last longer.
When you incubate nutrient agar plates in a warm place such as room temperature or a special incubator, the water in the plate tends to evaporate and condense on the srface of the plate. If you incubate the plate right side up then water drops form on the top of the plate and drip onto the plate causeing bacterial colonies to be spread, the spread very quickly in drops of water. But if you incubate the plate upside down the water is absorbed back into the plate as it condenses so there is never more than a very thin film of water across the surface of the plate so the bacteria can't spread more than they noramlly would.
The bacteria are very very light, and do not easily "fall off" of the plate. The actually adhere to the plate and each other, so they can grow even on a surface that is upside down, just like flies can land on the ceiling of your house.
You can, if the nutrient agar is clear, actually see the bacterial colony right through the plate, or you can turn the plate over and take the lid off and look at the colony. It is easeir to take the lid off, you can see much more clearly what the colony looks like than if you just look through the nutrient agar if it is clear.
After you have grown the bacteria on the plate it depends on what you want to do. If you are just going to throw the plate away, then no you don't need to refigerate it. But if you want to keep the plate for a while, wrap it in saran wrap so that it doesn't dry out, and stink up your fridge, and then refigerate it. But be advised some bacteria don't like being cold very much, but most don't mind it, they just grow much slower, or stop growing, and continue to grow when things warm up.Good luck!
Try the links in the MadSci Library for more information on Microbiology.