Relationships among selected species of viridans streptococci were studied by using deoxyribonucleic acid (DNA)-DNA hybridization and biochemical and serological tests. The results of hybridization experiments performed with DNA immobilized on membrane filters and index DNA made radioactive with [-H]thymidine indicated that ATCC 27335 (type strain), ATCC 27823, ATCC 9895, “ MG-intermedius” CDC SS 899, group F strain MGH 8153, and group F strain SFGH 879 are closely related to each other genetically, even though physiological tests indicated that this is a relatively heterogeneous group of organisms. The relative binding ratios for these six strains ranged from 60 to 99%, and the majority of values were greater than 85%, indicating that these strains should be considered members of the same species. Type strain ATCC 33399 (called ) is not a typical strain based on biochemical test results, and only moderate relatedness (41%) was observed in hybridization experiments performed with a typical strain (strain SS 429). I ATCC 10556 and II ATCC 10557 do not belong in the same species. A low relative binding ratio (13%) for these strains confirmed similar findings by previous investigators.


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