PLASMID STABILITY IN RECOMBINANT STRAINS OF LACTOCOCCUS-LACTIS SUBSP LACTIS DURING CONTINUOUS-CULTURE

The stability of plasmids introduced in Lactobacillus lactis IL1403 wa s followed in continuous cultures. Four strains with or without pIL205 and pIL252 or pIL253 (low or high copy number) were studied. pIL205 w as remarkably stable even after 180 generations, suggesting the presen ce of a partitioning system which is still unidentified. In contrast, pIL252 and pIL253 were unstable, in the presence as well as in the abs ence of pIL205. The multicopy plasmid pIL253 was nevertheless more sta ble than pIL252 (90% loss after 180 and 60 generations, respectively). The instability was not related to an incompatibility between pIL205 and pIL252 (or pIL253). The segregational instability of pIL252 and pI L253 plasmids could be explained by the loss of the resolvase gene dur ing their construction.