Re: Why should agar plates be dried before innoculation?

Hi Kate - Plates you make in the lab frequently solidify at room temperature, or may be placed at 4°C for storage until use.

When warmed to 37°C for incubation the agar gel doesn't hold the liquid as tightly, and some seeps free, either causing condensation on the lid, or droplets of water on the surface of the agar. You can get around the first issue by incubating the plates upside-down, something frequently done to avoid water on the lid dopping onto an incubating plate. The second issue is more problematic. Moisture on the surface of the agar can let bacteria swarm over the plate, making it difficult to obtain isolated colonies. Thus plates are frequently placed in a 60°C dryer for ~10 minutes or so to evaporate off excess moisture. The dryer needs to be clean so the plates don't become contaminated. You can place the agar side of the dish upside-down, propped over the edge of the top of the plate. Don't leave them in for too long, else you'll end up with a potato chip in the bottom of your plate.

Hope this helps..

-L. Bry, MadSci Admin