Plasmid DNA from lactic streptococci was subjected to electrophoresis on agarose gels. The plasmid profiles so obtained were strain specific and sufficiently stable to suggest their use in strain differentiation. A group of Streptococcus lactis strains, 712. 763 (ML3), 505 (C2) and 2031 (C2), found to have similar plasmid profiles, were shown to be closely related. Gene transfer by transduction and conjugation occurred between members of this group at frequencies comparable to those in homologous systems and temperate phages cross plated readily between their prophage cured derivatives.
Minor variations were, however, found between these four strains; slight differences in plasmid profiles, lysogenic status, prophage curability and temperate phage morphology were detected and it is suggested that these have evolved as a result of maintenance in different environments.