Science Buddies: "Ask an Expert"

Hi,

I received nutrient agar plates from Carolina Biological on 1/29 and 2/04. Both seem to have numerous small droplets of liquid on the agar. Should I be concerned? Does this indicate contamination? Or is it just water? I never opened the packages since arrival, and kept it refridgerated within a dark box. I need to use plates on Monday, and I'm concerned it will create complications with the E. coli i am growing..
Thanks!

It's probably condensation from the agar warming up and evaporating some of the liquid onto the lid. What I do when the plates are too wet is to lift the lids and leave them open a centimeter or so for half an hour to allow the liquid to evaporate. Obviously you don't want to contaminate the plates from nonsterile air so don't put them near an open window or where there are a lot of people or pets moving around.

If there are droplets of water condensed on the lid, you can take it off and give it a quick flip while holding it tightly by the edge in your fingers. Don't touch the inside of the lid, however.

Oh, a couple of things i forgot to mention. When you store plates and when you incubate them, always put them lid down. This is so any water that evaporates and condenses on the lid will not run down onto the agar.

Also, an hour or so before you are ready to spread your E coli, take the plates out of the refrigerator and allow them to warm up. If you can warm them to 37C, that's even better because the E coli will start growing quicker on warm agar.

One other thing--it's a good idea to tape the lids to the bases after you finish putting on the inhibitors so that they don't come off accidentally.

So it isn't I need to really worry about?
Oh it's not only on the lid, but on the agar itself as well. Is it ok? Maybe when I'm using the spreader when rolling out the broth culture, the liquid that was on will evaporate or something?

If the agar surface is too wet, do what i suggested--leave the lids open a bit in an undisturbed place until the surface liquid evaporates. Spreading won't do it. You can get pools of bacteria that make your lawn uneven. Also, E coli are motile. They have a flagellum, so they can swim around if there's too much water on the agar surface.

When you spread the 50 microliter inoculum, you want the surface to be just dry enough that the liquid is absorbed leaving the bacteria sitting on the surface in a uniform layer. That can't happen if the surface is wet.