The invention relates to an expression vector encoding a DNA defence system comprising polypeptides derived from Vibrio cholerae, or equivalent functional variants or homologues, conferring resistance to the maintenance of extra-genomic DNA to a recipient bacterial cell. The invention further relates to methods for obtaining an isolated bacterium, or a bacterial population capable of eliminating extra-genomic DNA, or of protecting against phage infection, and an isolated bacterium, or a product comprising an expression vector according to the invention.