Storage of Bacteria in microtitre trays - -80 or ???

[Hamish Scott]

We use a cDNA library that has 40,000 individual clones spotted into
96 well microtitre trays. The "working set" of trays has been thawed
two or three times (from -80) to produce high density filters 
containing the whole library for cDNA library screening. 
	Our problem is that the "working set" of plate seems to be 
dying regularly! It is stored in the -80 freezer with 10% glycerol
in the standard LB-amp media. The clones in the corners and at the 
edges of the plates seem to die last. 
	So, the question is, how do we store over 400 microtitre 
plates with bacteria in them so we don't have to keep making new 
copies from the "master plates"? Is it best to just keep them at
+4 -20 degrees rather than -80 if we are going to keep thawing them 
completely every few months? Any suggestions? Thanks.

Ciao
Hamish S. Scott
Division of Medical Genetics
University of Geneva
hscott at medsun.unige.ch