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Short answer: LB agar is Luria broth set with agar. One pre-mixed powder described by Addgene supplies, per litre, 5.0 g yeast extract, 10.0 g peptone from casein, 10.0 g sodium chloride and 12.0 g agar, which is 37 g of powder in 1 L of water. Other recipes change the agar and salt amounts, so “LB agar” is a family of media rather than one fixed formula.
Definition and scope
Addgene describes Luria broth (LB) as “a nutrient-rich media commonly used to culture bacteria in the lab.” Adding agar to the broth turns it into a gel that sets in a dish, giving the solid medium used for streaking, colony growth and selection. This page covers what goes into the medium and how published recipes differ. It does not cover growing any particular organism; for that, see culturing microorganisms and what an agar plate is.
What is in LB agar, per litre
Addgene’s protocol for pouring LB agar plates gives the following composition for pre-mixed LB agar powder, dissolved in 1 L of sterile water:
- 5.0 g yeast extract
- 10.0 g peptone from casein
- 10.0 g sodium chloride
- 12.0 g agar-agar
Together these make up 37 g of powder per litre. The same page gives a scaled example: 8.14 g of powder for 220 mL of medium. The peptone and yeast extract are the nutrient source, the sodium chloride sets the salt level, and the agar is the gelling agent.
How to make LB agar plates
According to Addgene’s protocol, the medium is autoclaved at a minimum of 121 °C and 20 psi for at least 30 minutes, with the cap or foil left loose rather than airtight and autoclave tape on the container. If a selective antibiotic is needed, it is added after autoclaving to agar that has cooled in a 60 °C water bath. Addgene gives 60 °C as a good temperature because the molten agar stays liquid there. The antibiotic is added from a 1000x stock. Working concentrations for ampicillin are covered in ampicillin working concentration, and the use of selective plates in bacterial transformation.
Addgene lists plate volumes of 5 to 10 mL for 60 x 15 mm dishes and 10 to 15 mL for 100 mm dishes. Poured plates settle for about 30 minutes, dry overnight and are stored at 4 °C in a bag with absorbent material. Addgene’s caution is direct: “You should not store your plates for longer than 1 month at any temperature.” Addgene also describes testing antibiotic plates by streaking a resistant and a non-resistant strain.
How recipes differ: two sourced examples
Comparing the two recipes below shows which numbers change between sources. Both are per litre of medium.
| Component (per litre) | Addgene LB agar powder | DSMZ MediaDive J1240, LB agar with 5% NaCl |
|---|---|---|
| Yeast extract | 5.0 g | 5 g |
| Casein peptone / tryptone | 10.0 g (peptone from casein) | 10 g (tryptone) |
| Sodium chloride | 10.0 g | 50 g |
| Agar | 12.0 g | 15 g |
| pH | Not given on the page | 7.0 |
The DSMZ entry is a deliberate high-salt variant, with five times the sodium chloride of the Addgene powder, so it is not a standard LB. That is the point of the comparison: the yeast extract and peptone amounts match, while sodium chloride and agar are the quantities that move. Two labs that both say “LB agar” can be pouring plates with different salt and different gel strength.
Where it matters for labs and supply decisions
Because the formula varies, a short specification avoids mismatched plates between people or between batches. A record of an LB agar plate can include:
- Whether it was made from a pre-mixed powder or from separate components, and the grams per litre of each.
- The sodium chloride level and the agar amount, the two values that differ between the sources above.
- The pH, where the recipe gives one (the DSMZ entry does; the Addgene page does not).
- Which antibiotic was added and at what stock strength.
- The dish size and the volume poured, using Addgene’s 5 to 10 mL and 10 to 15 mL ranges as the reference.
- The preparation date, since Addgene advises against storing plates for longer than 1 month.
Dishes themselves are covered in what a Petri dish is. For a different base medium with its own composition, see potato dextrose agar plates.
Common misunderstandings
“LB agar has one standard recipe”
The two sources compared above already differ on agar (12 g versus 15 g per litre) and, in the DSMZ high-salt variant, on sodium chloride (10 g versus 50 g). Name the recipe or the supplier’s formula when reporting results.
“Antibiotics go in before autoclaving”
In Addgene’s protocol the antibiotic goes in after autoclaving, once the agar has cooled to about 60 °C but is still liquid.
“Plates keep indefinitely in the fridge”
Addgene says plates should not be stored longer than 1 month at any temperature.
FAQ
What is LB agar made of?
In Addgene’s powder: yeast extract, peptone from casein, sodium chloride and agar-agar, at 5.0, 10.0, 10.0 and 12.0 g per litre. Other recipes vary the salt and agar amounts.
How to make LB agar plates?
Addgene’s protocol, in outline: dissolve the powder (37 g per litre) in sterile water, autoclave at 121 °C or above for at least 30 minutes with a loosely fitted cap, cool the molten agar in a 60 °C water bath, add any antibiotic stock, pour 10 to 15 mL into a 100 mm dish (5 to 10 mL for 60 mm), let the plates set for about 30 minutes, dry them overnight and store them at 4 °C. Follow your institution’s safety procedures and the label instructions on your powder.
How much agar is in LB agar?
Addgene’s powder contains 12.0 g per litre; the DSMZ LB agar entry uses 15 g per litre.
Sources
- Addgene, “Pouring LB Agar Plates”: https://www.addgene.org/protocols/pouring-lb-agar-plates/ (checked 2026-10-29).
- DSMZ MediaDive, medium J1240, “LB (Luria-Bertani) Agar with 5% NaCl”: https://mediadive.dsmz.de/medium/J1240 (checked 2026-10-29).
