Recent research in North America indicated that there is a wide diversity of Streptococcus suis (S. suis) strains that can be identified as pathogenic, opportunity or commensal strains. North American researchers found that Multilocus sequence type (MLST) was a better predictor of virulence than serotype. Some organisms rarely cause disease and simply colonize the pig while others are much more commonly associated with clinical disease. In the Netherlands and China, S. suis serotype 9 is increasingly associated with clinical disease. These Dutch researchers wanted to find out if all isolates of serotype 9 are equally able to cause disease. They collected tonsillar S. suis isolates from 6 herds not previously diagnosed with S. suis infections. They also collected clinical S. suis isolates from previously diseased pigs. They then confirmed the disease causing ability ( virulence) of a virulent type strain and one representative clinical isolate. In addition they confirmed the lack of disease causing ability of two carriage isolates by using a pig infection model.
Phylogenetic gene analysis of whole genome sequences of 124 isolates resulted in 10 groups of serotype 9 isolates. Two of these groups were almost uniquely populated by isolates that had come from clinical cases. 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.
Take Home Messages:
- S. suis serotype 9 population in the Netherlands is diverse and there are distinct virulence-associated lineages that could be discriminated based on the capsule locus sequence.
- These findings are in agreement with the recent North American studies that concluded that within a specific serotype there can be significant diversity with respect to virulence.
- S. suis serotype 9 are not all created equal.
Submitted by Clint Lichty DVM
Ref: Willemse N, van der Ark KCH, Stockhofe-Zurwieden N, Smith H, Picavet DI, van Solt-Smits C, Wisselink HJ, Schultsz C, de Greeff A. Clonal expansion of a virulent Streptococcus suis serotype 9 lineage distinguishable from carriage subpopulations. Sci Rep. 2019 Oct 28;9(1):15429. doi: 10.1038/s41598-019-51576-0.