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LB agar plates

Media & solutions

Standard selection plates for E. coli. About 25 mL of agar yields one ~10 cm plate; a 1 L bottle pours ~40 plates.

Recipe (1 L → ~40 plates)

• Tryptone — 10 g
• Yeast extract — 5 g
• NaCl — 5 g
• Bacto-agar — 15 g (1.5% w/v)
• Deionized water to 1 L

Procedure

1. Combine all components in a 2 L autoclavable bottle. Add a stir bar. Cap loosely.
2. Autoclave 121 °C, 25 min, liquid cycle.
3. Cool with stirring to ~55 °C (bottle is hot but holdable for several seconds against the back of your hand).
4. Add antibiotic from stock — see antibiotic stocks. Swirl well, avoiding bubbles.
5. Pour ~25 mL per 10 cm plate, working in a flame or laminar flow hood. Pop bubbles with a quick flame pass.
6. Let plates set on the bench for 30–45 min, lid slightly ajar to vent steam.
7. Stack inverted, wrap in plastic, and store at 4 °C. Use within ~1 month (carbenicillin/amp plates: 2–3 weeks).

Tips

• Pour at < 50 °C and you risk lumps; > 60 °C destroys antibiotic.
• If pouring under flame: keep hands above the bottle neck and lid above the plate to minimise contamination.
• Date and label plates with antibiotic + concentration on the side of the dish.
• Pre-warm plates to 37 °C for 20 min before plating to reduce condensation.