PLATE POURING

Plate pouring turns sterile, molten agar into a set of Petri dishes with a smooth solid surface for growing bacteria. Adding an antibiotic before pouring makes selective plates, where only resistant bacteria can grow.

WHAT YOU NEED?

• Sterile molten LB agar, cooled to about 50–55°C

• Antibiotic stock, if making selective plates

• Sterile empty Petri dishes

• Bunsen burner or hood, and a marker

STEPS

1. Label the bottoms of the plates with the medium, antibiotic and date.

2. Check the agar has cooled to about 50–55°C; the bottle should be hot but comfortable to hold.

3. Add the antibiotic to reach the working concentration (for example tetracycline at 20 µg/mL) and swirl gently to mix.

4. Working near the flame, lift each lid just enough to pour about 20–25 mL of agar, enough to cover the bottom.

5. You can also use a fumehood. Never bring flames in a hood.

6. Replace the lid straight away and gently swirl the plate to spread the agar evenly.

7. Leave the plates undisturbed until fully set, about 20–30 minutes.

8. Stack the plates upside down and store them at 4°C until needed.

TIPS

• If the agar is too hot, it kills antibiotics and fills plates with condensation; too cool, and it sets lumpy.

• Pop small bubbles by quickly passing the flame over the surface.

• Leave plates slightly open near the flame for a few minutes if they are very wet.