preservation of bacteria at -20deg.c - (Jun/29/2006 )

may be this is a very simple and stupid question to you but how to preserve bacteria at -20deg.c? for all bacterial is the method same?

15% glycerol final concentration for all the species I have worked with (enterobacteria nad streptomyctes mainly) but usually -70ºC is advised.

We store all bacteria in -80C.