dumb question about SOB media - (Sep/19/2005 )

I have been using SOB / SOC off and on for ages and ages, and I am just getting ready to make up a fresh batch for the first time in several years.

I got my protocol from an old e. coli transformation protocol in Gene back in 90

It says you are supposed to mix up the tryptone, YE, NaCl, KCl, and water...autoclave...

then, before you use it, add 1M MgCl2 and 1M MgSO4 that have been filter-sterilized.

this seems like a superfluous step to me. I have never understood why you can't just add the MgCl2 and the MgSO4 directly to the media prior to autoclaving, as both chemicals are perfectly autoclavable.

it works well for me, but it sure seems like I should be able to cut that step out and just add them directly. I think I'm going to try it that way.

CAN ANYONE THINK OF ANY GOOD REASON WHY THIS SHOULD BE ADDED SEPARATELY?

thank you

I've always done it this way as well (add the filter sterilized Mg compounds to the autoclaved media after cooling), though I do make a single 2M Mg2+ stock that is 1M MgCl2 and 1M MgSO4.

The only thing I can think of is that while the magnesium compounds are autoclavable on their own, they do not fare so well when in a rich mixture like yeast extract and tryptone, or perhaps their presence at that level (my recipe makes the media 20 mM with respect to Mg2+) in the rich media cause some trace ingredient to precipitate out or otherwise become unavailable.

I'd be interested in what you find if you try it -- if I had to guess, I'd say I don't think you'll find any difference.

I was looking for the same answer and I found your posting here...
The only thing I can say is that I did find some crystalization in SOB plates, where I autoclaved everything together...
I was also wondering if there is any reason why XL1-Blue cells grow best in LBM... Does anybody know why?
The last thing is: Does anybody know why in the Sambrook's book they say to add MgS04 to the SOB plates and MgCl2 to the liquid SOB?
Thanks,
Carla.