M9 medium is a minimal growth medium used for bacterial cultures. It has the advantage of being cheap and has a very low autofluorescence (when excited at 488nm) and also very low absorbance. M9 medium can be supplemented to produce higher growth rates or to allow growth of strains that require additives (e.g. thiamine or casamino acids).
A general discussion of minimal media for E. coli is given in this article (not online or in Pubmed): Pardee, A. B., F. Jacob, and J. Monod. 1959. The genetic control and cytoplasmic expression of "inducibility" in the synthesis of ß-galactosidase in E. coli. J. Mol. Biol. 1:165-178.