Survey of the extrachromosomal gene pool of Streptococcus mutans

Abstract

Fifty strains of S. mutans independently isolated from human dental plaque were examined for covalently closed circular plasmid DNA. CsCl-ethidium bromide centrifugation of [3H]thymidine-labeled, Sarkosyl-lysed cells revealed that 2 of the 50 strains contained plasmid DNA. The plasmid DNA from these strains was characterized by velocity and equilibrium centrifugation and by EM. The plasmids in these strains were virtually identical in size, with MW of 3.6 .cntdot. 106 and 3.7 .times. 106. Both were present to the extent of about 20 molecules/genome equivalent. Interlocked catenated dimeric molecules of each plasmid were readily detected by velocity sedimentation and EM. These plasmid-containing strains were compared with representative plasmid-free S. mutans strains by using such criteria as bacteriocin production, antibiotic susceptibility and hemolysis of mammalian erythrocytes. Although no correlation of phenotype to plasmid content was made, production of bacteriocin-like activity differed significantly between the 2 plasmid-containing S. mutans isolates. Although the plasmids in these 2 isolates appeared identical by the criteria of MW, presence of dimers and copy number, they appeared to be harbored by 2 distinct S. mutans strains.