An agar plate is a nutrient broth set with agar in a clean dish. How clean it needs to be depends on what you plan to grow on it.
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Warm agar poured thin, then left to set.
Agar plate, agar dish or petri dish?
A petri dish is the empty, shallow dish with a lid. The University of Utah calls 100 by 15 mm the most common size, and notes that glass dishes or even baby food jars also work.
An agar plate, also called an agar dish, is a petri dish holding growth medium that agar has set into a gel. The nutrients feed the microbes, and the agar keeps the medium semi-solid so that each microbe grows into its own colony.
Agar agar is the same seaweed gel sold for cooking, and it is what the kitchen recipe below uses.
Two kinds of plates
Kitchen plates grow whatever you swab from around the house. Kitchen Pantry Scientist makes them from beef bouillon, sugar and agar agar from the grocery store.
LB agar plates are what kits and lab protocols use for E. coli. They start from a medium sterilized under pressure, so only the strain you add grows.
Two recipes
Kitchen nutrient agar
LB agar
Water
1 cup
1 liter
Nutrients
1 bouillon cube or 1 tsp granules, and 2 tsp sugar
10 g peptone from casein, 5 g yeast extract and 10 g sodium chloride
Agar
1 tablespoon agar agar
12 g agar
Heat
Boil until the agar dissolves
Sterilize at 121 °C
Fill
Containers 1/3 to 1/2 full
About 25 mL per 100 mm plate
Keeps
Use within 2 to 3 days
4 to 6 weeks in the refrigerator
LB agar recipe for 250 mL, 500 mL and 1 liter
Batch
Premixed LB agar powder
Peptone from casein
Yeast extract
Sodium chloride
Agar
100 mm plates
250 mL
9.25 g
2.5 g
1.25 g
2.5 g
3 g
About 10
500 mL
18.5 g
5 g
2.5 g
5 g
6 g
About 20
1 liter
37 g
10 g
5 g
10 g
12 g
About 40
How to prepare agar powder
Weigh the powder for your batch. Addgene's premixed LB agar takes 37 g per liter, and the table above scales it down.
Put the powder in a glass bottle or flask that holds at least twice the volume of the medium, add the water and swirl to mix. Both Addgene and the University of Utah leave that much empty space so the medium can bubble without boiling over.
Heat it until the agar dissolves completely. The University of Utah says to keep boiling until the medium is clear, because agar that has not fully dissolved leaves the plates squishy or fragile.
For LB plates, sterilize the medium under pressure, as the next section explains. Kitchen plates can go straight to cooling and pouring.
Sterilizing without an autoclave
For kitchen plates, boiling is enough.Bitesize Bio microwaves agar for 2 to 4 minutes until it is clear and bubbling, stopping often so it doesn't boil over. Bitesize Bio lists those as non-autoclaved plates, so they are not sterile.
For LB plates, use pressure.Addgene autoclaves its LB agar at 121 °C for at least 30 minutes. At sea level, a stovetop canner at 15 psi reaches that temperature, as our pressure canner guide explains.
Heat the medium in a glass container that holds at least twice its volume, as the University of Utah advises, so it has room to bubble.
Plastic petri dishes come sterile and can't take the heat. Sterilize the medium in its bottle, then pour it into the dishes.
Wipe the counter with disinfectant or 10% bleach, and write on the bottom of each dish before you pour. The lid can turn, but the bottom stays put.
Let the medium cool for 20 to 30 minutes, until you can hold the container against your hand for a few seconds. Poured too hot, it fogs the lids with condensation.
Swirl gently without making bubbles, lift each lid just enough and pour about 25 mL, enough to cover the bottom of a 100 mm dish.
Leave the plates several hours to overnight to set. Then bag them, write the date on the bag and store them upside down in the refrigerator.
Sterile petri dishes
The University of Utah calls 100 by 15 mm the most common dish size. Both of these are that size, sterile and plastic.
EZ BioResearch sells a 25-pack with vented lids. Vabiooth's 100-pack has three vents in each lid, which Vabiooth says improves airflow and cuts condensation.
Petri dishes
EZ BioResearch sterile petri dishes, 100 mm
25 sterile polystyrene dishes, 100 by 15 mm, with vented lids.
Modernist Pantry's agar is sold for cooking, 50 g of high gel strength powder made from Gracilaria seaweed. It is the agar agar the kitchen recipe calls for.
To skip the cooking, EZ BioResearch sells 10 LB agar plates already poured, with sterile swabs.
Agar plates
EZ BioResearch LB agar plates and swabs
10 pre-poured 100 mm LB agar plates and 10 sterile 6-inch swabs, made in the USA.
Swabbed plates grow at room temperature within a few days. Do you need an incubator? covers when warmth helps and when it doesn't.
Tape each lid shut once something is on the plate, and bleach plates before they go in the trash. Our genetics experiments guide gives the bleach mix and soak time.
Treat every swabbed plate as unknown
You can't tell by looking which colonies are harmless. Keep swabbed plates taped shut and at room temperature, and never open one to smell or touch the growth.
Common questions
Heat water, a nutrient source and agar until the agar dissolves. Cool it until you can hold the container, pour about 25 mL into each 100 mm dish and let it set.
Yes. Boiling or microwaving works for plates you swab around the house, though they won't be sterile. For LB plates, a stovetop pressure canner at 15 psi reaches autoclave temperature at sea level.
Addgene's protocol uses 10 g peptone from casein, 5 g yeast extract, 10 g sodium chloride and 12 g agar per liter of water, sold premixed at 37 g per liter.
Yes, for kitchen plates. Kitchen Pantry Scientist uses 1 tablespoon per cup of water, with a bouillon cube and 2 teaspoons of sugar.
The University of Utah says plates keep 4 to 6 weeks upside down in the refrigerator. Kitchen Pantry Scientist uses its homemade plates within 2 to 3 days.
Yes. Addgene caps each plate after pouring and stacks the plates as it pours. Once they have set, the University of Utah bags them and stores them upside down in the refrigerator.
Leave swabbed plates taped shut at room temperature for a few days. A kit strain may call for a warm, steady temperature, so follow the kit's instructions.
