M9 medium

We used to prepare the plain M9 medium as follows:

900 ml sterile water

100 ml M9 salts stock solution

1 ml autoclaved 1 M MgSO4

0.1 ml autoclaved 1 M CaCl2

M9 stock solution (10x) is:

60 g Na2HPO4 x 7 H2O

30 g KH2PO4

5 g NaCl

10 g NH4Cl

add water to 1000 ml and autoclave

M9 medium also exists in various flavors:

M9-glu contains glucose: In the basic recipe take 800 ml sterile water and supplement with 100 ml 20% (w:v) sterile filtered glucose before adding M9 salts, magnesium and calcium salts.

M9-CA contains casamino acids: In the basic recipe take 800 ml sterile water and supplement with 100 ml 10% (w:v) autoclaved casamino acids (in water) before adding M9 salts, magnesium and calcium salts.

M9-CAglu contains both: In the basic recipe take 700 ml sterile water and supplement with 100 ml 20% glucose and 100 ml casamino acids before adding M9 salts, magnesium and calcium salts.

Warning: salts that are added last often tend to precipitate. I didn't bother about it too much as obviously it didn't influence bacterial growth. There might be a way to avoid precipitation (eventually adding magnesium and calcium first) but I didn't investigate this in detail.