Influence of dilution rate and cell immobilization on plasmid stability during continuous cultures of recombinant strains of Lactococcus lactis subsp. lactis.

The influence of dilution rate and cell immobilization on plasmid stability in recombinant strains of Lactococcus lactis subsp. lactis was investigated during continuous cultures. The studied strains, L. lactis IL2682 and IL2683, contained plasmids pIL9 (Lac+), pIL205 (CmR) and plasmids pIL252 (low copy number) and pIL253 (high copy number), respectively, that conferred resistance to erythromycin. Plasmid pIL205 was remarkably stable. Dilution rate did not affect the rate of loss of plasmids pIL252 and pIL253 significantly. Nevertheless, the loss of plasmid pIL253 was apparent after a further 21 generations when the dilution rate was decreased from 0.70 h-1 to 0.55 h-1. Cell immobilization in beads of kappa-carrageenan/locust bean gum improved plasmid stability by factors of 4.5 for pIL253 and 6.5 for pIL252. Thus, 10% of cells containing plasmids pIL252 or pIL253 were still present after 370 or 540 generations, respectively, compared with 50 or 210 generations in free cell cultures.