Pouring Agar Plates

Introduction

Agar plates are used for culturing microorganisms, and are used in myriad types of experiments. Agar is derived from kelp, and it added to a particular medium (such as Luria-Bertani broth or potato dextrose) to form a solid surface on which one may culture microorganisms such as bacteria, archaea, fungus, terrestrial plants and nematodes (worms). A 1-2% (g/100mL) solution of agar will be solid enough so that your culture will only grow laterally, but it is porous enough to allow the diffusion of nutrients in your medium to your hungry culture.

Reagents

Bunsen burner
In my experience, most Bunsen burners use natural gas, but a propane torch will or an alcohol lamp will work just as well. The drawback of an alcohol lamp is that you have to use a pretty concentrated solution of ethanol (like Everclear) to get a hot enough flame. Concentrated ethanol is heavily regulated in the United States, and therefore quite expensive for laboratory use, which is why I go with a propane torch.
autoclaved agar media
Latex gloves
You can also use dishwashing gloves; just make sure you sterilize the surface with unflavored vodka, ethanol, Purell or Lysol and make sure they've had time to evaporate before you work around an open flame.
sterile petri dishes

Procedure

1. Make sure your agar medium is sterile and liquid. See my section on autoclaving. The best thing to do is pull your media out of the autoclave, let it cool (swirling occasionally to keep it mixed) to the point that you can just barely handle it with latex gloves, and pour. Anything with agar solidifies around 55 degrees Celcius (body temperature is 37), so your medium will need to be warmer than that to pour smoothly. I've also noticed that the hotter I pour it, the less likely my plates are to be contaminated by errant spores. However, if it's too hot to handle it's likely hot enough to melt the plastic in your petri dishes. You can allow your agar to solidify and then microwave it to liquefy it, but (as I'm sure you already know) anything with a metal lid (like a mason jar) can't go in the microwave. Even without a metal lid, I've seen microwaving agar go really wrong (involving paramedics), so I avoid it.

2. While your medium is cooling, make sure you have a clean surface on which to work. Wiping your counter (or lab bench) down with ethanol (unflavored vodka, Everclear) or lysol will minimize the chances of contamination. Set up your plates and Bunsen burner so you can pour your plates between that crucial time when the agar is cooled just enough and when it's cooled to the point of solidifying.

3. Pour. Remove the lid from your vessel of autoclaved media and pass the top of the vessel through your flame. This will heat the air inside the vessel sufficiently to cause it to rise, lifting up (away from the sterile agar) whatever potential contaminants are hovering nearby. A thin layer of media that completely covers the bottom of the plate should be sufficient. Any thicker, and the plate will take longer to dry. If you manage to pour bubbles into your plate, quickly pass the flame of the bubbles. The heat should cause the bubbles to expand and pop, leaving you with a smooth agar surface on which to do your science. Just don't linger with the flame - you can very easily melt the plastic of the petri dish itself.

4. Wait for your agar to solidify. This should take less than an hour.

5. Invert and allow to dry. As the agar cools, water evaporates from it and condenses on the top lid of the petri dish. The water can then drip down onto the surface of your agar, making it very difficult to work. A simple way to avoid this is to simple flip the whole plate over. I usually allow the plates a day or two to dry before I use them.