Plasmid Stability Test

ruben.varela-calvino at ruben.varela-calvino at
Thu Apr 23 06:22:36 EST 1998

Hi everybody

I've my gen cloned into pET-5 vector and the recombinant construction
introduced into both BL21(DE3) and BL21(DE3)pLysS.
When I do the expression of the gen I take an aliquot of the culture to make
the plasmid stability test I have results that I don't understand:
1) with both cells, I always have more colonies (about 35%) on the plate with
antibiotic (ampicilim) than on the plate with no additives.
2) with the DE3 cells, on the plate with only IPTG I have a lot of colonies,
about 26% of the colonies I get on plates with Amp/no additives. This doesn't
happen with the (DE3)pLysS cells (only 6%, still too high I think). But on the
fourth plate, with both Amp/IPTG I have no colonies at all, that it's an
expected result.
Anybody knows how to read this results? Make they any sense?
Suggestions can be send me to ruben.varela-calvino at or posted into
the newsgroup.
Thanks in advance.

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