Clonal expansion of a virulent Streptococcus suis serotype 9 lineage distinguishable from carriage subpopulations

Abstract

Streptococcus suis is a porcine pathogen, causing severe invasive infections. S. suis serotype 9 is increasingly causing disease in Dutch and Chinese pig herds, but it is unknown whether all serotype 9 isolates are equally virulent and markers that can identify virulent strains are not available. Therefore, discrimination between virulent isolates and carriage isolates typically not associated with disease, is currently not possible. We collected tonsillar S. suis isolates from 6 herds not previously diagnosed with S. suis infections, and clinical S. suis isolates of previously diseased pigs. We confirmed the virulence of a virulent type strain and one representative clinical isolate, and the lack of virulence of two carriage isolates, in a pig infection model. Phylogenetic analysis of whole genome sequences of 124 isolates resulted in 10 groups, of which two were almost uniquely populated by clinical isolates. The population structure of S. suis serotype 9 appears highly diverse. However, analysis of the capsule loci sequences showed variation in a single region which fully correlated with a virulent genotype. Transmission electron microscopy suggested differences in capsule thickness between carriage and clinical genotypes. In conclusion, we found that that the S. suis serotype 9 population in the Netherlands is diverse. A distinct virulence-associated lineage was identified and could be discriminated based on the capsule locus sequence. Whilst the difference in virulence cannot be directly attributed to the DNA sequence, the correlation of capsule locus sequence with virulence could be used in the development of diagnostic tests to identify potential virulent S. suis serotype 9 in pigs.

Update on Streptococcus suis Research and Prevention in the Era of Antimicrobial Restriction: 4th International Workshop on S. suis

“…A systematic analysis of the data showed that the clinical isolates clustered mainly together in one group whereas the carriage isolates were distributed across 8 different groups. This suggested that virulent serotype 9 isolates are genetically very similar, whereas the carriage isolates are more heterogeneous [8], as was described before [10,64,79,84]. Based on our data, the heterogeneity was only found for the carriage isolates.…”

“…Nevertheless, other serotypes are increasing in importance in different countries, as is the case of serotype 9, particularly in some countries of Western Europe. By means of novel animal models and diagnostic tools, de Greeff et al (Appendix A) epidemiologically determined the population genetics of S. suis serotype 9 in The Netherlands [8]. Obtained data using comparative genome hybridization and whole genome sequencing suggest that clinical serotype 9 swine isolates are genetically very similar whereas serotype 9 isolates carried by healthy pigs are more heterogeneous.…”

“…To get more insight in the genetic population of S. suis serotype 9 in The Netherlands, an epidemiological study was performed [8]. Two populations of serotype 9 isolates were included for genotyping analysis: clinical isolates and isolates from healthy carrier piglets.…”

A critical review speculating on the protective efficacies of autogenous Streptococcus suis bacterins as used in Europe

“…Recent research has also highlighted the diversity of serotype 9 in Chinese S. suis isolates [11]. Furthermore, a collection of serotype 9 isolates in the Netherlands was shown to contain some strains with the ability to cause severe disease and others only inducing mild clinical signs which was confirmed in experimental infection of piglets [12]. Virulent serotype 2 strains in Europe generally carry the genes epf, mrp and sly encoding the extracellular factor, the muramidase-released protein and the hemolysin suilysin, respectively [8,9,13].…”

Streptococcus suis: An Underestimated Emerging Pathogen in Hungary?

“…The bacterial virulence-associated factors may be classified into the following sub-groups (of note, some genes probably may be classified into more than one categories due to their dual characteristics): (i) surface/secreted elements; (ii) protease enzymes; (iii) transcription factors/regulatory systems and (iv) other virulence determinants (transporters/secretion systems) [ 18 ]. The list of confirmed and putative virulence factors is extensive, but the most prominent and best-characterized virulence factors of clinical S. suis strains are surface and secreted elements: capsular polysaccharides, extracellular factor (EF; epf ), muramidase-released protein (MRP; mrp ) and a 38-kDa surface protein, suilysin (SLY) [ 19 , 20 , 21 , 22 ]. S. suis isolates are included a complex population consisting of heterogeneous strains and they may be classified into 35 serotypes (1–34, 1/2) to date, based on the principal virulence factor, the capsular polysaccharide antigen (CPS) [ 23 , 24 ].…”