The LB culture medium (for lysogeny broth) is a non-selective nutrient culture medium, initially used for bacterial culture.  LB media have become an industry standard for the cultivation of Escherichia coli. They have been widely used in molecular microbiology for the preparation of DNA plasmids and recombinant proteins. To this day, it remains one of the most widely used media for the life support and culture of recombinant Escherichia coli lines. There are various compositions of LB. Although they are different, they generally share some components that they have in common to promote the growth of species in culture. LB medium promotes luxurious growth of E. coli cells, as tryptone and yeast extract provide essential growth factors that E. coli cells would otherwise have to synthesize. The medium also contains essential electrolytes for transport and osmotic balance, thanks to the NaCl component.

The LB culture medium can be supplemented with antibiotics to make it selective. This allows the selection and selective culture of recombinant E. coli resistant to this antibiotic.