Genetic relationships deduced from emm and multilocus sequence typing of invasive Streptococcus dysgalactiae subsp. equisimilis and S. canis recovered from isolates collected in the United States

Abstract

Beta-hemolytic group C and G streptococci cause a considerable invasive disease burden and sometimes cause disease outbreaks. Little is known about the critical epidemiologic parameter of genetic relatedness between isolates. We determined the emm types of 334 Streptococcus dysgalactiae subsp. equisimilis isolates, and attempted emm typing of 5 Streptococcus canis isolates from a recent population-based surveillance for invasive isolates. Thirty-four emm types were observed, including one from S. canis. We formulated multilocus sequence typing (MLST) primers with six of the seven loci corresponding to the Streptococcus pyogenes MLST scheme. We performed MLST with 65 of the 334 surveillance isolates (61 S. dysgalactiae subsp. equisimilis isolates, 4 S. canis isolates) to represent each emm type identified, including 2 to 3 isolates for each of the 25 redundantly represented emm types. Forty-one MLST sequence types (STs) were observed. Isolates within 16 redundantly represented S. dysgalactiae subsp. equisimilis emm types shared identical or nearly identical STs, demonstrating concordance between the emm type and genetic relatedness. However, seven STs were each represented by two to four different emm types, and 7 of the 10 S. dysgalactiae subsp. equisimilis eBURST groups represented up to six different emm types. Thus, S. dysgalactiae subsp. equisimilis isolates were similar to S. pyogenes isolates, in that strains of the same emm type were often highly related, but they differed from S. pyogenes, in that S. dysgalactiae subsp. equisimilis strains with identical or closely similar STs often exhibited multiple unrelated emm types. The phylogenetic relationships between S. dysgalactiae subsp. equisimilis and S. pyogenes alleles revealed a history of interspecies recombination, with either species often serving as genetic donors. The four S. canis isolates shared highly homologous alleles but were unrelated clones without evidence of past recombination with S. dysgalactiae subsp. equisimilis or S. pyogenes.

FIG. 1.

emm type distribution observed among 334 S. dysgalactiae subsp. equisimilis isolates recovered from population-based surveillance of invasive streptococcal disease in areas of California and Georgia from 2002 to 2005.

FIG. 2.

Phylograms of murI (A); recP (B); and concatenated gki, gtr, murI, mutS, recP, and xpt (C) MLST data. We employed the neighbor-joining approach, using the Jukes-Cantor model. One thousand bootstrap replicates were performed on the same data by using the Paupsearch program to give the indicated frequencies of occurrence of bipartitions. seb, alleles exclusively found in a previous study of S. dysgalactiae subsp. equisimilis isolates. All known S. pyogenes and S. dysgalactiae subsp. equisimilis murI and recP alleles (as of June 2008) were also included in the analysis. Alleles with hyphens have been documented in both S. pyogenes and S. dysgalactiae subsp. equisimilis (B and C). Arrows show the S. dysgalactiae subsp. equisimilis recP alleles found during this study (B). In panel C, murI alleles are distinctly clustered in the indicated species-specific clusters (S. dysgalactiae subsp. equisimilis, S. canis, and S. pyogenes), while recP alleles from S. dysgalactiae subsp. equisimilis and S. pyogenes are not as distinctly separated. For the phylogram in panel C, the analysis included all STs from S. dysgalactiae subsp. equisimilis and S. canis found in this study and the S. pyogenes STs included in Table 2. For ST-Sc1 in panel C, the xpt6318 allele from another S. canis strain was employed since we were unable to amplify xpt from this strain. SP, S. pyogenes; SE, S. dysgalactiae subsp. equisimilis; SC, S. canis.