Plasmid transfer efficiency using Lactoccocus lactis strains depends on invasiveness status but also on plasmid copy number.

Lactic acid bacteria as Lactococcus lactis are used as vector for protein but also DNA delivery into intestinal cells in vitro and in vivo. For the plasmid delivery strategy, the plasmid copy number per bacteria (PCN) is thus of great importance. The aim of this paper is to determine the physiological conditions when PCN is the highest in the bacteria. PCN was characterized by qPCR in five different recombinant Lactococcus lactis strains, containing one (mono-) or two different plasmids (biplasmidic), at exponential or stationary phase. We showed that in all cases but one, PCN is higher at exponential than stationary phase. PCN seems to depend on (i) monoplasmidic or biplasmidic strain; (ii) origin of replication of the plasmid; and (iii) the DNA load of the bacteria. Then we studied plasmid transfer in vitro from recombinant L. lactis to eukaryotic COS-7 cells using culture at exponential or stationary phase. We showed that plasmid transfer can be improved in vitro by using bacteria at exponential phase.