DNA--DNA reassociation studies of Streptococcus constellatus with unusual 16S rRNA sequences

This study investigated the taxonomic status of unusual 'CI strains' of Streptococcus bacteria that exhibit atypical 16S rRNA sequences within the 'anginosus group' of streptococci. These CI strains were previously identified as hybridizing with probes from both S. constellatus and S. intermedius, making their classification uncertain. Researchers performed DNA-DNA reassociation studies on seven CI strains and compared them to type strains of the three anginosus group species. The CI strains demonstrated DNA-DNA reassociation values of 70% or greater with S. constellatus NCDO 2226T, with thermal stability values (ΔTm) less than 1°C, meeting both criteria for phylogenetic species identity. In contrast, reassociation with S. intermedius was only 52%, confirming separate species status. The results indicate that despite their unusual rRNA sequences, the CI strains represent a homogeneous group that belongs within S. constellatus rather than constituting a distinct species or subspecies. The authors conclude that biochemical heterogeneity within the anginosus group may result in additional rRNA subpopulations being recognized with further typing studies.

Key findings

• CI strains showed DNA-DNA reassociation values ≥70% with S. constellatus type strain NCDO 2226T, meeting species-level criteria
• ΔTm values <1°C indicated minimal genetic divergence between CI strains and S. constellatus, confirming phylogenetic identity
• CI strains demonstrated only 52% DNA-DNA reassociation with S. intermedius, establishing them as separate species
• CI strains form a homogeneous group despite unusual 16S rRNA sequences that hybridize with both S. constellatus and S. intermedius probes