Luria-Bertani broth, also known as LB broth or LB medium, is the most common liquid medium used to grow bacteria such as E. coli. It was named after two scientists who created it in the 1950s while they were studying phages. LB broth is an excellent medium because it is very efficient at stimulating growth and is suitable for many different organisms.
LB broth is categorized as a rich medium, meaning it contains all the nutrients such as peptides and peptones, vitamins, and trace elements needed for bacteria to proliferate. The recipe for the broth has changed little over the years and consists of three main ingredients: yeast extract, tryptone, and sodium chloride (salt). Yeast extract is basically a powdered form of the yeast found in the baking section of grocery stores. Tryptone is a pancreatic digest of the protein casein. Yeast extract and tryptone provide vitamins and amino acids, respectively, that the bacteria need to grow. Finally, sodium chloride is added to keep the broth at a certain ionic strength. LB broth has a strong yellow color and an extremely pungent smell that is reminiscent of Thai fish sauce.
The LB broth recipe is often altered depending on the bacteria and conditions. Salt amounts can vary anywhere between 0.5 grams and 10 grams. Bacteria that contain plasmids tend to grow best in broth that has between 5 and 10 g of salt. To promote faster growth, the medium can be supplemented with 0.1% glucose or 0.4% glycerol. Various cofactors may also need to be added to the broth if working with certain types of bacteriophages. For example, bacteriophage labmda requires an excess of magnesium in the broth for it to properly infect bacteria. Some recipes also keep the broth at a suitable pH of around 7.4 for ideal growth.
Biotechnology companies sell prepackaged LB broth in capsule and powder forms that just require adding water. This may save a small amount of time, but labs can save money instead by making the broth from scratch. A standard recipe is included below.
- Dissolve 10 grams of tryptone, 5 grams of yeast extract, and 10 grams of sodium chloride in 1 liter of deionized water. Adjust the pH of the solution to 7.4 using sodium hydroxide.
- Autoclave at 121°C for 20 min to sterilize the broth. Let cool.
- The broth can be stored sealed at room temperature. Antibiotics such as ampicillin are often added to the broth before adding bacteria to select for bacteria containing certain plasmids.
LB broth can also be turned into LB agar, a solid form of medium used to create plates for streaking out bacterial colonies. Simply add 15 grams of bacteriological agar during step one. Autoclave, let the mixture cool down to about 50 to 60°C, and pour the solution into petri dishes. Wait for about an hour for the broth to solidify. The plates can then be stored at 4°C in plastic bags. Antibiotics can also be added to LB Agar right before pouring into the dishes.