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

Niels Willemse,Constance Schultsz,Daisy I Picavet,Kees C H Van Der Ark,Hilde Smith,Astrid De Greeff,Norbert Stockhofe-Zurwieden,Henk J Wisselink,Conny Van Solt-Smits

doi:10.1038/s41598-019-51576-0

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.

Highlights

Streptococcus suis is an important pathogen associated with a wide range of diseases in pigs including meningitis, arthritis and sepsis[1,2], leading to major economic losses in the pig industry worldwide
We studied S. suis serotype 9 isolates obtained from the tonsils of healthy pigs from farms without overt S. suis specific disease for over 1 year and isolates obtained from clinically diseased pigs
We performed a systematic analysis of carriage isolates from the tonsils of healthy pigs and clinical S. suis serotype 9 isolates in the Netherlands

Summary

Introduction

Streptococcus suis is an important pathogen associated with a wide range of diseases in pigs including meningitis, arthritis and sepsis[1,2], leading to major economic losses in the pig industry worldwide. Serotype 2 is most often associated with clinical disease in pigs worldwide[5] as well as with zoonotic disease in humans[6,7,8], the proportion of serotype 9 among isolates from diseased pigs has increased substantially in the last two decades[9]. We showed that a clinical serotype 9 isolate was capable to induce severe disease in pigs after experimental infection, reconfirming its virulence, whereas carrier isolates did not cause severe disease. This indicates the presence of different pathotypes among S. suis serotype 9 isolates, which we further investigated using whole genome sequencing. We describe differences in the capsule loci between carriage and clinical isolates and propose the cpsK gene as a diagnostic target for the detection of virulent serotype 9 isolates

Methods

Results

Conclusion