Low-Salt LB 1.5% Agar Plates

MATERIALS

Agar (BD Bacto, dehydrated; BD 214010)

NaCl (Merck 1.06404)

Tryptone (BD 211705)

Yeast extract (BD 212750)

Autoclave

Bunsen burner (FIREBOY safety; Integra 144000) or match (see Step 5)

Erlenmeyer flask (2-L)

Magnetic stirrer

Magnetic stirring bar (50-mm)

Petri dishes (100-mm × 15-mm; Fisher Scientific FB0875713)

Place a magnetic stirring bar in a 2-L Erlenmeyer flask and add the following:

Mix briefly and autoclave for 20 min at 120°C and 15 psi.
Place the flask on a magnetic stirrer and stir at low speed.
- Avoid stirring too fast, as this generates bubbles that are fastidious to remove (see Step 5).
Wait for the liquid to cool down until you can barely touch the flask with your hand.
- This corresponds to a temperature of ∼60°C.
Pour the melted medium into Petri dishes (replacing the lid each time), aiming to pour about 25 mL per plate.
- With a little practice, you will be able to gauge this amount. Should bubbles form on the surface of the plates, quickly apply the flame of a Bunsen burner (or even that of a match) on the surface of the plate to pop the bubbles. Be careful not to melt the plastic of the dishes in the process.
Once the agar has hardened, turn the plates upside down, stacking them in piles.
- Inverting the plates prevents vapor condensation on the lids and stops H₂O droplets from falling on the surface of the agar.
Leave the plates for 3–4 d at room temperature to allow them to dry slightly.
Store the plates at 4°C.
- Most types of plates can be used for several months provided that they are protected from drying and contamination.