LB Agar Preparation & Plate Pouring Protocol

LB Agar is a widely used, nutrient-rich solid growth medium formulated from Lysogeny Broth supplemented with agar, providing a reliable surface for the cultivation and selection of Escherichia coli and other non-fastidious bacteria in molecular biology and microbiology laboratories.

Typical Use Case

Cultivation, cloning, and selection of E. coli and other non-fastidious bacteria on solid surface

Standard Concentration

35–40 g/L in distilled or deionised water (per manufacturer label; verify with L1528380 CoA)

Sterilisation Method

Autoclave at 121 °C, 15 psi for 15–20 minutes

Pouring Temperature

Cool molten agar to 50–55 °C before pouring to avoid condensation and plate distortion

Plate Storage

Sealed in bags at 2–8 °C, protected from light; use within 4 weeks

pH

7.0 ± 0.2 at 25 °C (verify on L1528380 CoA)

Materials Required

Before beginning, assemble the following:

• LB Agar powder (Aladdin Scientific, SKU: L1528380)
• Distilled or deionised water
• Appropriate glass bottles or Erlenmeyer flasks (autoclavable, at least 2× the liquid volume)
• Magnetic stir bar and hot plate/stirrer
• Autoclave
• Thermometer or temperature probe
• Sterile 90 mm (or required size) disposable Petri dishes
• Bunsen burner or biosafety cabinet (laminar flow hood)
• Parafilm or plate bags for storage
• Personal protective equipment: lab coat, heat-resistant gloves, safety goggles

Optional: Antibiotic stock solutions (e.g., ampicillin, kanamycin) if selective plates are required. Prepare filter-sterilised antibiotic stocks in advance.

Step-by-Step Preparation Protocol

Step 1 – Weigh and Dissolve

Consult the product label for SKU L1528380 to confirm the recommended mass per litre. Typically, weigh the appropriate amount of LB Agar powder and transfer to a clean, autoclavable glass bottle or flask. Add approximately 80% of the final target volume of distilled or deionised water. Stir with a magnetic stir bar on a hot plate until the powder is completely dissolved and the solution appears uniform. Add remaining water to reach the final volume.

Step 2 – Autoclave

Loosely cap or plug the flask with a foam stopper and autoclave tape. Sterilise by autoclaving at 121 °C, 15 psi for 15–20 minutes using a liquid cycle. Allow pressure to release naturally. Carefully remove the flask using heat-resistant gloves — the liquid will be very hot.

Step 3 – Cool to Pouring Temperature

Place the flask in a 50–55 °C water bath or allow it to cool on the bench, swirling occasionally to equilibrate temperature. Monitor with a thermometer. Do not allow the agar to solidify. If selective plates are required, add the appropriate antibiotic at this stage (at 50–55 °C to prevent thermal inactivation of antibiotics).

Step 4 – Pour Plates

In a biosafety cabinet or near a Bunsen burner (to maintain aseptic conditions), briefly flame the flask mouth and pour approximately 25–30 mL of molten LB Agar into each 90 mm Petri dish. Replace lid immediately. Allow plates to solidify on a level surface for 20–30 minutes at room temperature. Surface should be smooth and free of bubbles. If bubbles appear, briefly pass the flame of a Bunsen burner across the surface before the agar sets.

Step 5 – Dry the Plates (Optional but Recommended)

Once solidified, invert the plates and leave them slightly ajar in a biosafety cabinet or in an incubator at 37 °C for 20–30 minutes. This step removes excess surface moisture and reduces satellite colony formation.

Storage and Shelf Life

Poured plates: Stack inverted plates (agar side up) in sealed plastic bags to prevent dehydration and contamination. Store at 2–8 °C protected from light. Use within 4 weeks of preparation. Allow plates to equilibrate to room temperature (or 37 °C) before use to prevent condensation on the agar surface.

Unopened LB Agar powder (L1528380): Store in a cool, dry place, tightly sealed, away from moisture and direct light. Refer to the product label and Certificate of Analysis for the specific expiry date and storage conditions.

Molten agar: If not used immediately after autoclaving, LB Agar can be kept molten in a 50–55 °C water bath for several hours. Prolonged holding at elevated temperature may degrade heat-labile components; re-sterilisation of previously autoclaved media is not recommended.

Quality Control and Troubleshooting

Quality control checks:

• Sterility: Incubate 2–3 uninoculated plates at 37 °C for 24–48 hours. No colony growth should be observed.
• Growth promotion: Inoculate a plate with a known E. coli strain (e.g., DH5α) and incubate at 37 °C overnight. Expected result: robust, well-isolated colonies.
• pH: Verify dissolved agar solution pH is 7.0 ± 0.2 at 25 °C before autoclaving using a calibrated pH meter.

Common problems and solutions:

Problem	Likely Cause	Solution
Contaminated plates	Non-aseptic pouring technique or compromised autoclave cycle	Verify autoclave validation; improve aseptic technique; check autoclave tape and biological indicators
Agar too thick or too thin	Incorrect powder weight or volume error	Reweigh and recheck calculation; verify with product label for L1528380
Bubbles on surface	Pouring too quickly or at too-high temperature	Cool further before pouring; flame surface before agar sets
Excess condensation	Plates stored without inversion or not dried before storage	Invert plates during storage; perform drying step before bagging
Poor colony growth	Overheating of media, degraded antibiotic, or expired powder	Check expiry of L1528380; verify antibiotic concentration and addition temperature

Frequently asked questions

What is the recommended dissolution concentration for LB Agar (L1528380)?

The standard preparation typically falls in the range of 35–40 g/L; however, always refer to the specific product label and Certificate of Analysis for SKU L1528380 to confirm the exact recommended concentration, as formulation details may vary between lots.

Can I add antibiotics to LB Agar for selective plating?

Yes. After autoclaving and cooling the molten LB Agar to 50–55 °C, add filter-sterilised antibiotic stock solution at the appropriate working concentration (e.g., ampicillin at 100 µg/mL or kanamycin at 50 µg/mL are common working concentrations). Adding antibiotics above 60 °C risks significant thermal inactivation.

How long can poured LB Agar plates be stored, and what are the storage conditions?

Poured plates should be stored inverted in sealed plastic bags at 2–8 °C, protected from light, and used within 4 weeks of preparation. Plates that show signs of contamination, excessive drying, or cracking should be discarded. Equilibrate plates to room temperature before use.

Why do my LB Agar plates show satellite colonies around transformant colonies?

Satellite colonies typically indicate that the antibiotic (commonly ampicillin) has been degraded around productive colonies, allowing sensitive cells to grow. Solutions include: drying plates thoroughly before use, reducing incubation time, using carbenicillin in place of ampicillin (more stable), or increasing antibiotic concentration. Ensure plates are not older than 4 weeks and that the antibiotic was added at the correct temperature.

Is LB Agar suitable for organisms other than E. coli?

LB Agar supports the growth of many non-fastidious gram-negative and gram-positive bacteria. However, it is not suitable for fastidious organisms requiring enriched or specialised media (e.g., blood agar or chocolate agar). Always confirm media suitability for your specific organism before use.

References

1. Bertani, G. (1951). Studies on lysogenesis. I. The mode of phage liberation by lysogenic Escherichia coli. Journal of Bacteriology, 62(3), 293–300. [Original description of LB (Lysogeny Broth) medium] PMID:14888646
2. Sambrook, J., & Russell, D. W. (2001). Molecular Cloning: A Laboratory Manual (3rd ed.). Cold Spring Harbor Laboratory Press. [Standard reference for LB media preparation protocols]
3. Aladdin Scientific. Product Specification and Certificate of Analysis: LB Agar, SKU L1528380. [Consult current CoA for exact dissolution concentration, pH specification, and storage conditions]

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.