Protocol Online logo
Top : New Forum Archives (2009-): : General Lab Techniques

SOB media - (Feb/26/2009 )

I am making some SOB media to make competent cells.
I was in doubt as to why my protocol says add sterilized MgCl2 separately and JUST BEFORE USE.
Does that mean that this medium is good for one time use only?

I make SOC stock and use it for a long time, so I don't think this would be the case, but I don't get the "JUST BEFORE USE" part.



minutes before you want to use SOB medium, add sterile MgCl2 solution.

Provided you sterile technique is good, and with the help a refrigerator, you can use the same bottle of medium several times.

MgCl2 solution has to be added after the autoclaving. Due to the amount of MgCl added to the medium, the Mg ions it will form a precipitate if autoclaved. And once Mg percipitate forms it is hard to redissolve.


we normally add the MgCl after autoclaving, make batches of 1ml and freeze this until we need them


I have found no need to add the MgCl2 just before use; I simply add it after the autoclaved solution has cooled. I make a 500 ml bottle of media, add the appropriate amount of magnesium solution, and keep it on my shelf at room temperature to be used as needed. Never had a problem...


One can add MgCl2 after the SOB is autoclaved and cooled. If you use it in a sterile way, you can use it as long as it lasts, or you risk contamination and have to make it again.