Plasmid expression in stressed bacteria

Abstract

Plasmids are small double-stranded DNA molecules that are present in a high variety of bacteria. They usually encode non-essential proteins, such as antibiotic resistances or toxins, which can become an advantage during the course of infection. Even though the behaviour of bacteria under stress has been widely studied, the dynamics underlying the expression and copy number of plasmids under these conditions have yet to be characterized. We have cloned into two plasmids with different nominal copy numbers a construct that contains a fluorescent protein gene (YFP) regulated by an inducible promoter. After transformation of these plasmids into two different Bacillus subtilis strains, one with a chromosomal integration of the same construct and a wild-type one, we have studied the changes in fluorescence levels produced by variations in stress conditions. Flow cytometry results show an increased level of YFP expression from the high-copy plasmids in cells under stress, in comparison to those that grow in common LB medium. Transformed cells give rise to two populations with different fluorescence values; the first one presents similar expression to that of cells with only a chromosomal copy of the construct while the second shows higher fluorescence levels.