Conjugative mobilization as an alternative vector delivery system for lactic streptococci.

Due to the current variability in applying polyethylene glycol-mediated protoplast transformation to lactic streptococci, a study was undertaken to assess the feasibility of conjugative mobilization as an alternative method for vector delivery. By using the broad-host-range conjugative plasmid pVA797, the partially homologous cloning vector pVA838 was successfully introduced into various strains of Streptococcus lactis, Streptococcus cremoris, Streptococcus lactis subsp. diacetylactis, Streptococcus thermophilus, and Streptococcus faecalis. Frequencies ranged from 10(-2) to 10(-6) transconjugants per recipient. Both pVA797 and pVA838 were acquired intact, without alteration in functionality. Also, the shuttle vector pSA3, which shares partial homology with pVA797, was mobilized via conjugation. The use of S. lactis LM2301 as the intermediate donor allowed the use of physiologic and metabolic characteristics for recipient differentiation. The construction of a vector containing a "DNA cassette" conferring mobilization and the resolution, segregation, and stability of the cointegrates, pVA797, pVA838, and pSA3, are also reported.