Comparative genomic analyses provide clues to capsule switch in Streptococcus suis

Streptococcus suis (S. suis) is a major bacterial pathogen in swine industry and also an emerging zoonotic agent. S. suis produces an important extracellular component, capsular polysaccharides (CPS). Based on which, dozens of serotypes have been identified. Through virulence genotyping, we uncovered the relatedness between proportions of SS2, SS3 and SS7 strains despite their differences in serotypes. Multi-locus sequence typing (MLST) was used to characterize whole S. suis population, revealing that there is capsule switch between S. suis strains. Importantly, capsule switch occurred in SS2, 3 and 7 strains belonging to CC28 and CC29, which is phylogenetically distinct from the main CC1 SS2 lineage. To further explore capsule switch in S. suis, comparative genomic analyses were performed using available S. suis complete genomes. Phylogenetic analyses suggested that SS2 strains can be divided into two clades (1 and 2), and those classified into clade 2 are colocalized with SS3 and SS7 strains, which is in accordance with above virulence genotyping and MLST analyses. Clade 2 SS2 strains presented high genetic similarity with SS3 and SS7 and shared common competence and defensive elements, but are significantly different from Clade 1 SS2 strains. Notably, although the cps locus shared by Clade 1 and 2 SS2 strains is almost the same, a specific region in cps locus of strain NSUI002 (Clade 2 SS2) can be found in SS3 cps locus, but not in Clade 1 SS2 strain. These data indicated that SS2 strains appeared in CC28 and CC29 might acquire cps locus through capsule switch, which could well explain the distinction of genetic lineages within SS2 population.