However, recently several large human outbreaks of S. suis have been described in China [3, 4], and Thailand
, whilst S. suis meningitis has become endemic in Vietnam [6, 7], suggesting that isolates that are more virulent to humans have emerged. The S. suis population is very heterogeneous as different serotypes, phenotypes, and genotypes are found. To date 33 capsular serotypes have been described for S. suis [2, 8] of which serotypes 1, 2, 7, 9, and 14 are most frequently isolated from diseased pigs in Europe . In Northern America, besides these serotypes, serotypes 3 and 8 are frequently 17-AAG in vivo isolated from diseased animals [10, 11]. On European farms, it was shown that up to 81% of healthy animals carried one or more serotypes simultaneously and different genotypes of the same serotype could be isolated at one timepoint from the same animal . Different phenotypes of serotype 2 were described that differ in their virulence; strains can be differentiated by NU7441 molecular weight protein expression of virulence markers muramidase released protein (MRP), extracellular factor (EF) and suilysin (SLY)
[13, 14]. Besides variation in protein expression observed among S. suis PF-6463922 strains, large heterogeneity also exists in gene composition [10, 15–17]. Recently, the genome sequence of S. suis serotype 2 strain P1/7 became available  enabling whole genome typing techniques for S. suis. In the present study, we performed oligonucleotide-based comparative genome
hybridization (CGH) using the genome sequence of strain P1/7 to evaluate gene conservation and diversity among S. suis strains. Fifty-five well characterized S. suis strains of various serotypes were analyzed in this CGH study. Results from CGH were clustered, and correlated with MLST data, SB-3CT serotyping results, and virulence of strains. We showed that groups of S. suis isolates can be identified by their own unique profile of putative virulence genes and regions of difference. Besides, a core genome for S. suis was defined. Methods Bacterial strains and growth conditions Bacterial isolates are described in Table 1. S. suis strains were grown on Columbia agar blood base plates (Oxoid Ltd., London, United Kingdom) containing 6% (vol/vol) horse blood. Cultures were grown in Todd-Hewitt broth (Oxoid). Escherichia coli was grown in Luria Broth (Oxoid) and plated on Luria Broth Agar (Oxoid). S. suis isolates used in this study were serotyped using the slide-agglutination test  before they were used in the study (Table 1). Expression of three virulence markers, MRP, EF, and SLY [19, 20] was confirmed for all isolates by Western blot analysis  using monoclonal antibodies against MRP, EF , or SLY  (Table 1). Table 1 Characteristics of bacterial strains used in this study.