A family of isogenic L. lactis strains were constructed and used as gene donors in transfer experiments. They carried the same genetic markers (antibiotic resistance genes were used) but on several different supports:
a self-transmissible plasmid;
a broad host range nontransmissible plasmid, having either a low or a high copy number;
Recipient strains were either a suitably marked L. lactis strains, used for experiments carried out in fermenters or in cheese, or Enterococcus faecalis strains of human origin, used in the mouse digestive tract.
Gene transfer from L. lactis, which is efficient under optimal conditions, was very infrequent in the 3 model systems examined. It was always detected with a self-transmissible plasmid, rarely with a mobilisable plasmid and never with chromosomally carried genes. These observations suggest that the potential risks associated to gene transfer from genetically modified lactic bacteria are probably small.