SPREAD PLATING

Spread plating spreads a small volume of liquid culture evenly over the whole agar surface. Each cell grows into its own colony, which is useful for screening a library or counting how many bacteria were in a sample.

WHAT YOU NEED?

• Agar plates (plain or selective)

• Liquid culture or diluted sample

• Micropipette and sterile tips

• Sterile cell spreader (disposable, or glass/metal sterilized with ethanol and a flame)

• Bunsen burner

STEPS

1. Label the plates on the bottom.

2. Pipette a small volume, usually 50–100 µL, onto the centre of the agar.

3. Sterilize the spreader (or open a sterile disposable one).

4. Lift the lid and slightly move it on the empty parts of the agar to cool it down

5. Gently spread the liquid across the whole surface while turning the plate until soaked.

6. Leave the plate near the flame or in the hood, lid slightly open, until the liquid has soaked in.

7. Close, turn upside down and incubate overnight.

TIPS

• If you flame a spreader dipped in ethanol, let it cool before touching the agar, or it will kill the cells.

• Keep the ethanol beaker well away from the flame.

• Too much liquid makes colonies run together; stick to 100 µL or less.