When endothelial cells were infected with S. suis S735 serotype 2, only isolated bacteria and small chains were visualized (Fig. 1b). Additional serotype 2 strains (90-1330, 99-1539B, 89-4223, 89-999, 31533)
were also found to adhere markedly less to endothelial cells when compared with nontypeable isolates (data not shown). Streptococcus suis strains were analysed using transmission electron microscopy and ruthenium red staining for the presence of a polysaccharide capsule. ABT-263 cell line Figure 2c–j shows that nontypeable S. suis 1079277, 1078212, 1185293, and 1148795 did not express a dense capsule. The three other nontypeable strains of S. suis (1097925, 1077009, and 1079506) were also devoid of capsule (data not shown). By contrast, S. suis S735 (Fig. 2a and b) as well as two other serotype 2 strains tested (data not shown) possessed a thick and dense capsule. We then evaluated whether capsule expression alters the cell surface hydrophobicity
of S. suis. As shown in Table 2, nontypeable S. suis 1079277, 1097925, 1078212, learn more 1185293, 1148795, 1077009, and 1079506 showed a high percentage of cell surface hydrophobicity (≥52%). On the contrary, the hydrophobicity of all S. suis serotype 2 strains was <29%. In view of the above results, we investigated the capacity of autoaggregation of S. suis strains. Table 2 shows that nontypeable isolates were able to autoaggregate to various extents, while the serotype 2 strains could not. All the tested S. suis strains possessed cell-associated DPP IV activity. However, only six strains of S. suis RVX-208 (S735, 1078212, 1079277, 1097925, 1185293, and 1148795) showed subtilisin-like protease activity after 4 h of incubation. Extending the incubation to 24 h did not modify the result. Finally, we compared biofilm formation by nontypeable and serotype 2 strains of S. suis. Figure 3 shows that nontypeable isolates had the capacity to form a dense biofilm into wells of the polystyrene plate while serotype 2 strains had
no such property. Only slight variations were observed regarding the growth capacity of all S. suis strains (data not shown). Streptococcus suis is a Gram-positive cocci that possesses cell wall antigenic determinants related to Lancefield group D (Facklam, 2002). Based on the capsular composition, currently, there are 35 serotypes described for S. suis species (Gottschalk & Segura, 2000; Messier et al., 2008). Serotyping is an important step in the routine diagnostic procedure for S. suis infections. Different procedures have been described, but most laboratories use the coagglutination technique (Higgins & Gottschalk, 2001; Costa et al., 2005). Although the incidence of nontypeable isolates is in general low, their isolation is reported in the literature (Higgins & Gottschalk, 2000; Wei et al., 2009). Because very few data are available regarding the properties of nontypeable pathogenic S.