Population structure and antimicrobial resistance of invasive serotype IV group B Streptococcus, Toronto, Ontario, Canada

Abstract

We recently showed that 37/600 (6.2%) invasive infections with group B Streptococcus (GBS) in Toronto, Ontario, Canada, were caused by serotype IV strains. We report a relatively high level of genetic diversity in 37 invasive strains of this emerging GBS serotype. Multilocus sequence typing identified 6 sequence types (STs) that belonged to 3 clonal complexes. Most isolates were ST-459 (19/37, 51%) and ST-452 (11/37, 30%), but we also identified ST-291, ST-3, ST-196, and a novel ST-682. We detected further diversity by performing whole-genome single-nucleotide polymorphism analysis and found evidence of recombination events contributing to variation in some serotype IV GBS strains. We also evaluated antimicrobial drug resistance and found that ST-459 strains were resistant to clindamycin and erythromycin, whereas strains of other STs were, for the most part, susceptible to these antimicrobial drugs.

Keywords: Canada; Streptococcus agalactiae; Toronto; antimicrobial resistance; bacteria; bacterial infection; group B Streptococcus; invasive bacterial disease; multilocus sequence typing; population structure; serotype IV; streptococci; whole-genome sequencing.

Figure 1

Inferred genetic relationships of invasive serotype IV group B Streptococcus (GBS), Toronto, Ontario, Canada. A) Neighbor-joining phylogenetic tree constructed by using the concatenated sequences of the 7 gene loci (sdhA, adhP, tkt, glcK, atr, pheS, and glnA) used in the multilocus sequence typing (MLST) scheme for GBS. Each circle represents a single MLST sequence type (ST); circle colors differentiate the 6 STs found among strains in our collection and their sizes are proportional to the number of isolates. B) Neighbor-joining phylogenetic analysis based on concatenated sequences of 316 nonredundant single-nucleotide polymorphism (SNP) loci identified by whole-genome sequencing relative to the core genome ST-452 reference strain NGBS572, which shows further diversity within ST-452. C) Diversity within ST-459 serotype IV GBS shown by neighbor-joining phylogenetic analysis based on concatenated sequences of 526 nonredundant SNP loci relative to the core genome of ST-459 reference strain NGBS061. Scale bars indicate nucleotide substitutions per site.

Figure 2

Susceptibility to erythromycin, clindamycin, and tetracycline among serotype IV group B Streptococcus and sequence types (STs), Toronto, Ontario, Canada. All ST-459 strains and 1 ST-291 strain were resistant to erythromycin; we detected ermTR and ermT genes in all erythromycin-resistant strains (Technical Appendix). Resistance to clindamycin was observed only among ST-459 strains. Resistance to tetracycline was common among all STs except ST-452 and correlated with presence of the tetM gene in isolates (Technical Appendix). Scale bars indicate nucleotide substitutions per site.