Genetic transformation in some cariogenic Streptococcus milleri.

Only one of six Streptococcus milleri strains, known to produce dental caries in gnotobiotic rats, was found to be spontaneously transformable. At optimal competence (after 60 min incubation in transformation medium) DNA uptake by this strain, CR 287, was rapid; maximum DNA was taken up by cells within 15 min of its addition to give a transformation frequency of 1 x 10(-2) transformants per colony-forming unit (cfu). With post-optimally competent cells, the rate of DNA uptake was markedly decreased, although a transformation frequency similar to that of optimally competent cells was obtained. For example, after 240 min incubation in transformation medium the cells required approximately 90 min to reach maximum DNA uptake and gave a transformation frequency of 6 x 10(-3) transformants per cfu. Cultures retained the ability to give their maximum transformation frequency for at least 4 h, but only if a DNA-cell contact time of 2 h was used. Strain CR 287 and S. milleri strain NCTC 10707, a transformable non-cariogenic strain, produced competence factor (CF). The CF of CR 287, but not that of NCTC 10707, induced high competence in the non-spontaneously transformable cariogenic S. milleri strain NCTC 11169. Like S. sanguis, S. milleri must therefore produce CFs of different constitutional types. The three transformable strains could be transformed by plasmid shuttle cloning vector pVA838 DNA.