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We examined a collection of 151 strains of the viridans type of streptococci, which were isolated mainly from human oral cavities and included several reference strains, in an attempt to revise and improve the taxonomy of this group of bacteria. Our examinations included determinations of a high number of biochemical and physiological characteristics and serological reactivity. The resulting data revealed several hitherto unrecognized characters of taxonomic significance, and several of the species can now be more accurately defined. A diagnostic key to the taxa is presented. Strains previously identified as Streptococcus sanguis could be divided into two clearly distinct species, Streptococcus sanguis sensu stricto (type strain, ATCC 10556) and a new species, Streptococcus gordonii (type strain, ATCC 10558). Streptococcus mitis was divided into two biovars, consisting of strains possessing group O antigens and strains possessing group K antigen. The group of strains assigned to Streptococcus anginosus was biochemically and serologically heterogeneous, but the data did not allow natural subdivisions. Based on the results of this study, emended descriptions of the species Streptococcus oralis, S. mitis, and S. sanguis are provided. The classification resulting from this study is in complete agreement with previously published genetic data.

The type strain of Streptococcus mitis Andrewes and Horder 1906, 712 is strain NCTC 3165. This strain was arbitrarily designated as the type strain on the 1980 Approved Lists of Bacterial Names before adequate documentation of its characteristics. It is not an authentic strain of Andrewes and Horder; subsequent examination has indicated that it almost certainly is not representative of the species described by these authors, nor is it representative of strains widely recognized as S. mitis. We propose replacement of strain NCTC 3165 with strain NCTC 12261 as the type strain of S. mitis.