1. Melt or autoclave medium-agar.

2. Let it cool down to 55 °C.

3. Add X-gal (5-Bromo-4-chloro-3-indolyl-D-galactopyranoside in DFM) (and antibiotics) to the final concentration of 20 mg/ml.

4. Pour plates.

II. Pouring top agar plates

1. Pre-make bacteria plates without X-gal.

2. Melt or autoclave medium-agar.

3. Let it cool down to 55 °C.

4. Add X-gal (5-Bromo-4-chloro-3-indolyl-D-galactopyranoside in DFM) (and antibiotics) to the final concentration of 20 mg/ml.

5. Pour a thin layer of agar medium on top of the pre-made plates.

III. Spreading

1. Dilute X-gal (20 mg/ml) stock to 2 mg/ml with DFM.

2. Spread 100 ml per 10 cm pre-made plate.

3. Allow plates to dry.