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LB (Lysogeny broth or Luria-Bertani) is the most commonly used growth medium for E. coli culture. LB is a rich medium that contains tryptone, yeast extract and sodium chloride. It promotes fast growth of E. coli in culture by fulfilling all nutritional requirements, thus suitable for all regular molecular biology applications including molecular cloning, plasmid amplification, and recombinant protein production. Several versions of the original recipe formulated by Giuseppi Bertani in 1951 are now in common use: Miller broth, Lennox broth, Luria broth. They differ in NaCl concentration.

SOB (Super Optimal Broth) medium is a nutrient-rich microbial growth medium, designed by Douglas Hanahan in 1983. SOB or its glucose-supplemented version SOC is often used during transformation of E. coli competent cells with plasmids (added in transformed cells after heat shock). It results in better survival and higher transformation efficiency as compared to regular LB medium. It contains 2% Bacto-tryptone, 0.5% Bacto-yeast extract, 0.05% Sodium chloride, 2.5 mM KCl, and 10 mM MgCl2. A 1000 ml SOB medium is prepared by dissolving 20 g bacto-tryptone, 5 g bacto-yeast extract, 0.5 g sodium chloride and 2.5 ml of 1M KCl in water to a final volume of 1000 ml. Medium is sterilized by autoclaving. MgCl2 (5 ml of 2M MgCl2) is added just before use.