Abstract
Streptococcus suis serotype 2 is the main cause of zoonotic S. suis 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 S. suis 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 S. suis 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 S. suis population isolated from humans (0.598) was smaller than of the population isolated from pigs (0.765, P = 0.05) indicating that the S. suis population isolated from infected pigs was more diverse than the S. suis population isolated from human patients. S. suis 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 S. suis infection, irrespective of the ST and EF encoding gene type of S. suis strains.
Highlights
Streptococcus suis is an emerging human pathogen
Human Isolates During the period 1986–2007, 24 S. suis isolates obtained from human patients with meningitis were submitted to the National Reference Laboratory of Bacterial Meningitis (NRLBM)
We observed significant differences between the S. suis populations isolated from human patients and from diseased pigs in the Netherlands
Summary
The number of human S. suis infections reported worldwide has increased significantly in the past years, with most cases originating in Southeast Asia [1]. Human S. suis infection is acquired through exposure to contaminated pigs or pig derived products. S. suis infections are causing huge losses in pig production worldwide. Multiple putative virulence factors of S. suis have been identified [2,3,4,5], including the polysaccharide capsule [6,7], none of these have been shown to be essential for infection. The epf gene encoding Extracellular Protein Factor (EF), is a virulence-associated gene and is used as a virulence marker an epf mutant was not severely attenuated in virulence in experimental infection [8,9]. Muramidase Released Protein (MRP) and the hemolysin (suilysin), have been used as virulence markers [8,11]
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