This review traces the history of the human, nonhemolytic, or viridans, streptococci and describes improvements in their taxonomy wrought by study of their biochemical profiles and analysis of their nucleic acids. The goal was to define species on the basis of genetic relationships and to describe these species by their phenotypic characteristics so that they can be easily identified. This method has resulted in the division of some species. Streptococcus mutans has been divided into four species, two of which are common in humans. Three more mutans group species are indigenous to animals. Conversely, S. constellatus, S. intermedius, and "S. milleri" have been combined under S. anginosus. S. mitis (or "S. mitior") can be well-defined and includes S. sanguis II. There is genetic heterogeneity within S. sanguis, but the species is usually easy to identify. There is also some heterogeneity in S. bovis, but most human isolates are genetically related. Discussions of the taxonomy of these species are accompanied by descriptions of the characteristics by which these streptococci can be identified. Among these species are potential pathogens which should be suspected in cases of endocarditis and purulent infections of liver, brain, and other tissues.
It is proposed to confer species rank to the genetically distinct streptococci that have been considered subspecies of Streptococcus mutans Clarke: S . mutans subsp. rattus Coykendall, S. mutans subsp. cricetus Coykendall, and S . mutans subsp. sobrinus Coykendall. S , mutans is defined to exclude phenotypically similar bacteria that have deoxyribonucleic acid (DNA) guanine plup Oytosine contents appreciably different from 36 to 38 mol% and/or that do not demonstrate DNA base-sequence homology with the type strain. Streptococci that resemble S. mutans but that are quite disparate in molecular constitution are here regarded as comprising four new species: S. rattus (Coykendall) Deoxyribonucleic acid (DNA) hybridization differences in lactate dehydrogenases among (32) and the analysis of enzymes by serological three of the genospecies in respect to the enand electrophoretic methods have provided new zyme's kinetic response to pyruvate (5). He also approaches to bacterial taxonomy and fostered demonstrated electrophoretic differences in a molecular concept of species (27, 34). Thus, mannitol-and sorbitol-1-phosphate dehydroone may now define a species as a bacterium genases (4). Using serological methods, London that has a unique overall sequence of nucleo-found that aldolases of "S. mutans" were hetertide bases in its chromosome and a complement ogeneous and concluded that "S. mutans" inof enzymes characteristic of that species. Put cluded "divergent groups" of organisms (26). simply, we expect members of a species to not Heterogeneity has also been observed in polyonly "look alike" and "act alike," but also to be saccharide-synthesizing enzymes (7, 28) and in constructed of, and controlled by, molecules invertase (J. M. Tamer et al., J. Dent. Res., vol that are alike. Thus, members of a species 54, special issue A; Abstr. 53rd Gen. Session should be molecularly homologous. Bacteria Inter. Assoc. Dent. Res., 1975). S. mutans genthat are molecularly disparate should not be ospecies correlate well with the serological called by the same species name. groups described by Bratthall (2). Additional Some organisms reported to resemble Strep-serogroups found by Perch et al. (30) seem to tococcus mutans Clarke 1924 (8) are phenotypi-represent variants within the genospecies (12). cally quite similar (6,16,17,30) but are molecu-Differences have been observed in cell wall carlarly heterogeneous. Analysis of their DNA bohydrates among some genospecies (24), and base-sequence similarities has shown the exist-some morphological differences have been reence of four genetic groups (ll), which we call ported (13). "genospecies," a term introduced by Ravin (31).It is clear that "S. mutans" is actually a DNA from strains within each genospecies hy-group of streptococci, but, because of the overall bridized almost completely, but DNA from biochemical similarity of all S. mutans strains, strains of different genospecies hybridized it was proposed to conserve the nomenspecies poorly (10,ll). Furthermore, intergroup hybrid "Streptoc...
We studied strains of an unusual streptococcus that superficially resembles Streptococcus sanguis but has fibrils that are arranged in lateral tufts. These strains were originally isolated from human throats and oral cavities and have been referred to previously as "Streptococcus sanguis I," the "CR group," and the "tufted-fibril group.'' Until now, insufficient phenotypic data have been available to allow reliable differentiation of these strains from other viridans streptococcal species, particularly the species in the S . sanguis group. Recently, workers have proposed a scheme of phenetic tests that is based on 4-methylumbelliferyl-linked substrates and conventional biochemical tests and allows the tufted-fibril group to be differentiated; these organisms differ from other viridans species in being able to hydrolyze arginine but not esculin and in producing a-L-fucosidase but not P-glucosidase or alkaline phosphatase. These data, together with the results of our DNA-DNA hybridization experiments and the unusual ultrastructure of the tufted-fibril strains as determined by electron microscopy, demonstrate that these organisms represent a new species, for which the name Streptococcus crista is proposed. The DNA base composition is 42.6 to 43.2 mol% G+C. The type strain is strain CR311 (= NCTC 12479).In recent years the use of nucleic acid hybridization and sequencing techniques has helped clarify the taxonomy of the oral viridans streptococci, resulting in the description of several new species (11,20,21), emended species descriptions (11, 18), and the division of these bacteria into the following three species groups: the Streptococcus mutans group, the Streptococcus oralis group, and the Streptococcus salivarius group ( 5 , 10, 16). Several of the studies involving DNA-DNA hybridization (5, 7) have highlighted a group of strains that superficially resemble Streptococcus sanguis, have been isolated from human throats and oral cavities, and have tufts of fibrils located in a lateral position on the cell surface (8). The majority of S. sanguis strains isolated from coronal plaque have short ( 4 4 nm) peritrichously arranged sparse fibrils, but workers have isolated a small group of strains that have lateral fibrils organized in a very dense tuft on only one side of the cell (8, 9). These tufted S . sanguis strains are known to coaggregate specifically with Corynebacterium matruchotii both in vitro and in vivo, forming characteristic corn cob configurations in mature plaque (12, 14, 15), a property that is not found in the peritrichously fibrillar S . sanguis strains (8).The tufted strains hydrolyze arginine but not esculin, do not produce alkaline phosphatase or ferment raffinose (9,
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