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 stab ility in recombinant strains of Lactococcus lactis subsp. lactis was i nvestigated during continuous cultures. The studied strains, L. lactis IL2682 and IL2683, contained plasmids pIL9 (Lac(+)), pIL205 (Cm-R) an d plasmids pIL252 (low copy number) and pIL253 (high copy number), res pectively, that conferred resistance to erythromycin. Plasmid pIL205 w as remarkably stable. Dilution rate did not affect the rate of loss of plasmids pIL252 and pIL253 significantly. Nevertheless, the loss of p lasmid pIL253 was apparent after a further 21 generations when the dil ution rate was decreased from 0.70 h(-1) to 0.55 h(-1). Cell immobiliz ation in beads of kappa-carrageenan/locust bean gum improved plasmid s tability 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 3 70 or 540 generations, respectively, compared with 50 or 210 generatio ns in free cell cultures.