Luria broth (LB) is the most widely used liquid growth medium for culturing bacteria in research laboratories. Its primary job is to grow Escherichia coli and other common bacteria quickly and reliably for experiments in molecular biology, genetics, and biochemistry. If you’ve encountered the term in a protocol or a textbook, chances are it’s being used to multiply bacteria so researchers can harvest DNA, produce proteins, or test how cells behave under different conditions.
What LB Broth Is Made Of
The standard recipe calls for just three ingredients dissolved in one liter of distilled water: 10 grams of tryptone, 5 grams of yeast extract, and 10 grams of sodium chloride (table salt). Each ingredient has a specific nutritional role. Tryptone is a mix of small protein fragments (peptides and amino acids) derived from digesting milk protein, and it serves as the main nitrogen and carbon source bacteria feed on. Yeast extract supplies vitamins, trace minerals, and additional amino acids. Sodium chloride maintains the salt balance so bacterial cells don’t burst or shrivel from osmotic stress.
Before use, the pH is adjusted to 7.0 (neutral) and the broth is sterilized by autoclaving at 120°C for 25 minutes. This kills any contaminating organisms so only the bacteria a researcher intentionally adds will grow in the flask.
Growing Bacteria for Everyday Lab Work
The most common use of LB is simply multiplying bacteria overnight so there are enough cells to work with the next morning. A researcher picks a single bacterial colony from a plate, drops it into a tube or flask of LB, and places it in a shaking incubator at 37°C. By the next day, the culture is dense with billions of cells. This “overnight culture” is the starting point for nearly every downstream experiment, from extracting DNA to studying gene expression.
LB is popular for this role because it’s cheap, easy to prepare, and supports fast, reproducible growth of E. coli, the workhorse organism of molecular biology. Most labs keep pre-made bottles on hand at all times.
Plasmid DNA Isolation
One of the biggest reasons researchers reach for LB is to grow bacteria that carry plasmids, which are small circular pieces of DNA used to shuttle genes between organisms. After growing a plasmid-carrying E. coli culture overnight in LB, you can crack the cells open and purify the plasmid DNA. For high-copy-number plasmids (the most common type), as little as 1.5 mL of an overnight LB culture produces generous yields of DNA in what’s called a “miniprep.” That purified DNA can then be used for cloning, sequencing, or transferring genes into other cells.
For low-copy or single-copy plasmids, LB alone may not produce enough DNA, and researchers sometimes switch to richer media. But for routine plasmid work, LB remains the default.
Antibiotic Selection After Transformation
When researchers insert a new plasmid into bacteria (a process called transformation), they need a way to tell which cells actually took up the DNA and which didn’t. Plasmids are designed to carry an antibiotic resistance gene for exactly this purpose. After transformation, bacteria are grown in LB spiked with a specific antibiotic. Only cells carrying the plasmid survive, because the resistance gene protects them. Cells without the plasmid die off.
The two most common antibiotics used this way are ampicillin (at 100 micrograms per milliliter) and kanamycin (at 50 micrograms per milliliter). LB works well as the base medium here because it supports healthy growth of the surviving bacteria without interfering with the antibiotic’s activity.
Protein Production
LB is also a standard starting medium for producing recombinant proteins, meaning proteins encoded by genes that were artificially inserted into bacteria. Researchers grow transformed E. coli in LB, then trigger the cells to start making large quantities of the target protein. Studies have shown that protein expression in LB can match or exceed that of more specialized media for certain applications, making it a practical first choice before optimizing with richer alternatives.
Solid Media for Plates
Adding agar (a seaweed-derived gelling agent) to LB broth before autoclaving turns it into a solid medium. Poured into petri dishes, these “LB agar plates” are used to grow individual bacterial colonies on a surface. This is essential for isolating single clones after a transformation, counting colony-forming units, or streaking out a freezer stock to check viability. The nutritional composition is identical to the liquid form; only the physical state changes.
Limitations of LB
Despite its popularity, LB is not a rich medium. It contains no added sugar, so bacteria quickly exhaust the limited carbon sources in the tryptone and yeast extract. E. coli grown in LB typically reach a maximum density that’s lower than what richer formulations can support. For experiments requiring very high cell densities or maximum protein yields, labs often switch to alternatives like Terrific Broth or Super Broth, which contain additional carbon sources like glycerol and higher concentrations of yeast extract.
LB also has salt-level variations that matter for certain protocols. The most common version (sometimes called Miller) uses 10 g/L of sodium chloride, but a lower-salt version called Lennox uses only 5 g/L. The reduced-salt formulation is preferred when using certain antibiotics that are sensitive to salt concentration or when growing bacterial strains that perform poorly in higher osmotic conditions.
Why It’s Called LB
The abbreviation “LB” is widely assumed to stand for “Luria-Bertani,” after microbiologists Salvador Luria and Giuseppe Bertani. Bertani developed the recipe in the early 1950s for experiments on bacterial viruses. However, Bertani himself later clarified that “LB” originally stood for “Lysogeny Broth,” since he designed it to study lysogeny, the process by which a virus integrates its DNA into a bacterial chromosome. The “Luria-Bertani” name stuck anyway through decades of informal use, and today you’ll see both names on commercial labels.