<p>Abstract</p> <p>Background</p> <p><it>Streptococcus suis </it>is a zoonotic pathogen that causes infections in young piglets. <it>S. suis </it>is a heterogeneous species. Thirty-three different capsular serotypes have been described, that differ in virulence between as well as within serotypes.</p> <p>Results</p> <p>In this study, the correlation between gene content, serotype, phenotype and virulence among 55 <it>S. suis </it>strains was studied using Comparative Genome Hybridization (CGH). Clustering of CGH data divided <it>S. suis </it>isolates into two clusters, A and B. Cluster A isolates could be discriminated from cluster B isolates based on the protein expression of extracellular factor (EF). Cluster A contained serotype 1 and 2 isolates that were correlated with virulence. Cluster B mainly contained serotype 7 and 9 isolates. Genetic similarity was observed between serotype 7 and serotype 2 isolates that do not express muramidase released protein (MRP) and EF (MRP<sup>-</sup>EF<sup>-</sup>), suggesting these isolates originated from a common founder. Profiles of 25 putative virulence-associated genes of <it>S. suis </it>were determined among the 55 isolates. Presence of all 25 genes was shown for cluster A isolates, whereas cluster B isolates lacked one or more putative virulence genes. Divergence of <it>S. suis </it>isolates was further studied based on the presence of 39 regions of difference. Conservation of genes was evaluated by the definition of a core genome that contained 78% of all ORFs in P1/7.</p> <p>Conclusions</p> <p>In conclusion, we show that CGH is a valuable method to study distribution of genes or gene clusters among isolates in detail, yielding information on genetic similarity, and virulence traits of <it>S. suis </it>isolates.</p
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