LB agar (plates) recipe
LB plates for selecting E. coli transformants — LB plus 1.5% agar. About 20–25 mL per 90 mm dish.
🧮 Volume calculator
| Component | Amount | Final concentration (1X) |
|---|---|---|
| Tryptone | 5 g | 1% |
| Yeast extract | 2.5 g | 0.5% |
| NaCl | 5 g | 1% |
| Agar | 7.5 g | 1.5% |
| Water (distilled) | to final volume |
Steps
- Dissolve everything except agar and check pH 7.0.
- Add agar (it will not dissolve yet) and bring to volume.
- Autoclave (121 °C, 20 min) — the agar melts.
- Cool to 50–55 °C, add antibiotic and pour.
Good to know
- Store set plates upside down at 4 °C (antibiotic plates ≈1 month).
- Pass a flame briefly over the surface to pop bubbles.
FAQ
What if I add antibiotic while too hot?
Heat destroys ampicillin and others — add at about 55 °C, when you can hold the bottle.
This is a widely used standard composition. Amounts depend on the hydrate form and purity of your reagents — check the formula weight (FW) on the bottle and follow your institution's safety rules and the manufacturers' instructions.