Conjugative co-transfer of lactose and bacteriophage resistance plasmids from Streptococcus cremoris UC653

When conjugative transfer of lactose-fermenting ability (Lac) was observed between Streptococcus cremoris UC653 (donor) and S. lactis MG1363 Sm (recipient), 70% of the Lac⁺ transconjugants had acquired total resistance to phage 712 and propagated phage C2 at a lower efficiency and with a reduced plaque size. Plasmid analysis of transconjugants combined with curing experiments showed that the Lac and phage resistance markers were associated with plasmids of 26 and 50 MDa, respectively. Some transconjugants contained a large plasmid of either 77 or 83 MDa which coded for both Lac and phage resistance. The phage resistance mechanism did not act at the adsorption stage and was not affected by incubation at 37°C.