Delineation of Streptococcus dysgalactiae, its subspecies, and its clinical and phylogenetic relationship to Streptococcus pyogenes

Abstract

The taxonomic status and structure of Streptococcus dysgalactiae have been the object of much confusion. Bacteria belonging to this species are usually referred to as Lancefield group C or group G streptococci in clinical settings in spite of the fact that these terms lack precision and prevent recognition of the exact clinical relevance of these bacteria. The purpose of this study was to develop an improved basis for delineation and identification of the individual species of the pyogenic group of streptococci in the clinical microbiology laboratory, with a special focus on S. dysgalactiae. We critically reexamined the genetic relationships of the species S. dysgalactiae, Streptococcus pyogenes, Streptococcus canis, and Streptococcus equi, which may share Lancefield group antigens, by phylogenetic reconstruction based on multilocus sequence analysis (MLSA) and 16S rRNA gene sequences and by emm typing combined with phenotypic characterization. Analysis of concatenated sequences of seven genes previously used for examination of viridans streptococci distinguished robust and coherent clusters. S. dysgalactiae consists of two separate clusters consistent with the two recognized subspecies dysgalactiae and equisimilis. Both taxa share alleles with S. pyogenes in several housekeeping genes, which invalidates identification based on single-locus sequencing. S. dysgalactiae, S. canis, and S. pyogenes constitute a closely related branch within the genus Streptococcus indicative of recent descent from a common ancestor, while S. equi is highly divergent from other species of the pyogenic group streptococci. The results provide an improved basis for identification of clinically important pyogenic group streptococci and explain the overlapping spectrum of infections caused by the species associated with humans.

Fig 1

Radial minimum evolution tree based on concatenated sequences of seven housekeeping genes that shows the phylogenetic positions of the species groups within the Streptococcus genus. The tree was constructed from concatenated sequences of the 95 strains from this study combined with sequences of 420 strains, primarily from the mitis group, from the study by Bishop et al. (8), and sequences extracted from the whole genomes of 40 Streptococcus strains.

Fig 2

Minimum evolution tree based on concatenated sequences of seven housekeeping genes and showing the position of the 112 strains of S. dysgalactiae, S. equi, S. canis, and S. pyogenes. The strains within the defined clusters are color labeled: S. dysgalactiae subsp. equisimilis subcluster 1, gray; S. dysgalactiae subsp. equisimilis subcluster 2, purple; S. dysgalactiae subsp. dysgalactiae subcluster 1, blue; S. dysgalactiae subsp. dysgalactiae subcluster 2, turquoise; S. pyogenes, yellow; S. canis, green; S. equi subspecies, orange. Open symbols, arrows, and designations indicate the positions on the tree of the type strains. S. dysgalactiae subsp. equisimilis and S. dysgalactiae subsp. dysgalactiae are both divided into two subclusters. Triangles indicate isolates from this study, while squares indicate sequences extracted from whole-genome sequences. Bootstrap values for major branches are shown.

Fig 3

Minimum evolution tree of concatenated sequences of the seven housekeeping genes of the S. dysgalactiae subsp. equisimilis and S. dysgalactiae subsp. dysgalactiae isolates with relation to Lancefield antigen, emm type, isolation site, and hemolysis. Bootstrap values for major branches are shown.

Fig. 4.

Minimum evolution tree of the 16S rRNA gene sequences from the 76 strains of S. dysgalactiae from this study as well as 79 sequences from both subspecies of S. dysgalactiae extracted from the Ribosomal Database Project 10 (RDP10) database. Sequences from this study are shown as triangles: S. dysgalactiae subsp. equisimilis subcluster 1, gray; S. dysgalactiae subsp. equisimilis subcluster 2, purple; S. dysgalactiae subsp. dysgalactiae subcluster 1, blue; S. dysgalactiae subsp. dysgalactiae subcluster 2, turquoise. Sequences from GenBank assigned as S. dysgalactiae subsp. equisimilis are shown as green squares, while sequences assigned to S. dysgalactiae subsp. dysgalactiae are shown as red squares. Bootstrap values for major branches are shown.