Streptococcus downei sp. nov. for Strains Previously Described as Streptococcus mutans Serotype h

Abstract

Strains of streptococci originally isolated from the dental plaque of monkeys (Macaca fascicularis) and designated as Streptococcus mutans serotype h were compared with the other species of the mutans streptococcus group. Despite the close resemblance noted previously between these strains and Streptococcus sobrinus, closer examination revealed several important differences. Strains of serotype h ferment mannitol but not sorbitol, melibiose, inulin, or raffinose, do not produce hydrogen peroxide, and are unable to grow in the presence of bacitracin at 2 units per ml. They exhibit a distinct polypeptide pattern by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and possess several antigens absent from S. sobrinus as revealed by Western blotting (immunoblotting). Virtually identical polar lipid patterns were observed by two-dimensional thin-layer chromatography for both serotype h strains and S. sobrinus, although on the basis of long-chain fatty acid analysis by capillary gas-liquid chromatography, the former could be distinguished by the presence of a peak tentatively identified as cyclopropane acid (cis-9, 10-methyleneoctadecanoate (ΔC 19:0). Deoxyribonucleic acid (DNA)-DNA hybridization studies by both the S1 nuclease and renaturation rate methods showed that serotype h strains differ from S. mutans, S. sobrinus, Streptococcus cricetus, Streptococcus rattus, Streptococcus ferus, and Streptococcus macacae. On the basis of these data, we believe that S. mutans serotype h strains represent a distinct species for which the name Streptococcus downei is proposed. The DNA base composition is 41 to 42 moles percent guanine plus cytosine. The type strain is strain MFe28 (NCTC 11391T), which is cariogenic in monoassociated germfree rats.