Differences in the Population Structure of Invasive <em>Streptococcus suis</em> Strains Isolated from Pigs and from Humans in the Netherlands

<div><p><em>Streptococcus suis</em> serotype 2 is the main cause of zoonotic <em>S. suis</em> infection despite the fact that other serotypes are frequently isolated from diseased pigs. Studies comparing concurrent invasive human and pig isolates from a single geographical location are lacking. We compared the population structures of invasive <em>S. suis</em> strains isolated between 1986 and 2008 from human patients (N = 24) and from pigs with invasive disease (N = 124) in the Netherlands by serotyping and multi locus sequence typing (MLST). Fifty-six percent of pig isolates were of serotype 9 belonging to 15 clonal complexes (CCs) or singleton sequence types (ST). In contrast, all human isolates were of serotype 2 and belonged to two non-overlapping clonal complexes CC1 (58%) and CC20 (42%). The proportion of serotype 2 isolates among <em>S. suis</em> strains isolated from humans was significantly higher than among strains isolated from pigs (24/24 vs. 29/124; P<0.0001). This difference remained significant when only strains within CC1 and CC20 were considered (24/24 vs. 27/37,P = 0.004). The Simpson diversity index of the <em>S. suis</em> population isolated from humans (0.598) was smaller than of the population isolated from pigs (0.765, P = 0.05) indicating that the <em>S. suis</em> population isolated from infected pigs was more diverse than the <em>S. suis</em> population isolated from human patients. <em>S. suis</em> serotype 2 strains of CC20 were all negative in a PCR for detection of genes encoding extracellular protein factor (EF) variants. These data indicate that the polysaccharide capsule is an important correlate of human <em>S. suis</em> infection, irrespective of the ST and EF encoding gene type of <em>S. suis</em> strains.</p>