Plasmid heterogeneity in Streptococcus cremoris M12R: effects on proteolytic activity and host-dependent phage replication.

Examination of single colony isolates from a culture of Streptococcus cremoris M12R revealed a high degree of variability in plasmid deoxyribonucleic acid composition. Fifty percent of the M12R population displayed proteolytic activity and harbored a 13-Mdalton plasmid (pLR2013). This plasmid was not present in proteinase-deficient variants isolated from the culture, which provided correlative evidence for linkage of proteinase activity to pLR2013. Four percent of the M12R population demonstrated resistance to phage m12r X M12. This resistance was identified by restriction and modification activities against m12r X M12 phage, which was dependent on the presence of a 20-Md plasmid, pLR1020. Loss of restriction and modification activities was observed upon curing of pLR1020. In conjugal mating studies with Streptococcus lactis ME2, transfer frequency of lactose-fermenting ability to a restriction and modification-deficient variant of M12R was 10(2)-fold higher than to a variant exhibiting restriction and modification activities. The data provided evidence for restriction and modification activities in select S. cremoris M12R variants that are linked to pLR1020 and restrict both the plaquing ability of phage and efficiency of plasmid transfer by conjugation.