Elsevier

Veterinary Microbiology

Volume 139, Issues 3–4, 18 November 2009, Pages 310-317
Veterinary Microbiology

Serotype distribution and production of muramidase-released protein, extracellular factor and suilysin by field strains of Streptococcus suis isolated in the United States

https://doi.org/10.1016/j.vetmic.2009.06.024Get rights and content

Abstract

Streptococcus suis is an important swine pathogen and a zoonotic agent. Differences in virulence have been noted among the 33 described serotypes, serotype 2 being considered the most virulent. In this study, we aimed at assessing the serotype distribution and the production of virulence-associated markers by strains recovered from diseased pigs in the United States (U.S.). Results showed that among the 100 strains evaluated, serotype 3 (20% of the isolates) and serotype 2 (17%) were the most prevalent. We then investigated the presence in these isolates of the genes sly, epf and mrp, encoding the virulence-associated markers suilysin (SLY), extracellular factor (EF) and muramidase-released (MRP) protein, respectively. The effective production of the markers by the strains was also verified. Results showed that the presence of the gene did not always correlate with actual expression of the respective protein. In the case of MRP, this was due, in most cases, to frameshift mutations at the 5′ end of the gene resulting in premature stop codons. The most prevalent phenotypes among U.S. strains were MRP+EFSLY (40%) and MRPEFSLY+ (35%). Serotype distribution greatly differed from that reported in several European countries, as did the production of virulence markers, particularly for serotype 2. On the other hand, our results for the U.S. S. suis isolates are similar to those reported for Canadian strains, suggesting a common status in North America.

Introduction

Streptococcus suis is a major agent of meningitis, septicemia, and other diseases in pigs and is also a zoonotic agent (Gottschalk et al., 2007). Among the 33 S. suis serotypes, serotype 2 is the most frequently associated with disease (Higgins and Gottschalk, 2006). Important differences in virulence also exist among strains of the same serotype (Berthelot-Herault et al., 2005, Quessy et al., 1995, Vecht et al., 1992). The capsule, a fibronectin/fibrinogen binding protein, a serum opacity-like factor and modifications of the cell wall lipoteichoic acids and peptidoglycan have been reported to be involved in the virulence of this pathogen (Baums et al., 2006, Chabot-Roy et al., 2006, de Greeff et al., 2002, Fittipaldi et al., 2008a, Fittipaldi et al., 2008b, Smith et al., 1999).

Suilysin (SLY, encoded by gene sly) is a haemolysin which has a cytotoxic effect, might affect complement deposition and might allow penetration of S. suis into deeper tissues (Chabot-Roy et al., 2006, Jacobs et al., 1994, Lalonde et al., 2000, Lun et al., 2003, Vanier et al., 2004). Together with SLY, the LPXTG-protein known as muramidase-released protein (MRP, 136 kDa, encoded by gene mrp) and the secreted protein extracellular factor (EF, 110 kDa, encoded by gene epf), have been used as phenotypic markers of virulence. Indeed, there is a positive association between the presence of these proteins and virulence in Eurasian strains of S. suis (Gottschalk et al., 2007, Vecht et al., 1991). In these continents, serotype 2 MRP+EF+SLY+ strains are mainly isolated from diseased pigs showing severe clinical signs of disease while MRPEFSLY strains have been frequently isolated from healthy pigs (Allgaier et al., 2001, Vecht et al., 1992). On the other hand, serotype 2 MRP+EF* strains, which produce high molecular weight variants of EF, have been reported to be less virulent for young pigs (Vecht et al., 1992). In addition, avirulent strains possessing MRP, EF and suilysin have not been reported. However, some European and most Canadian virulent isolates do not produce any of these 3 factors (Berthelot-Herault et al., 2000, Gottschalk et al., 1998, Quessy et al., 1994, Segers et al., 1998).

Despite the fact that many studies addressing the prevalence of MRP, EF and SLY have been carried out with European and Canadian isolates, the prevalence of MRP, EF and SLY in the United States (U.S.), the largest North American swine producer, remains poorly known. Indeed, only one study has conducted a phenotypic evaluation of the presence of MRP and EF in selected strains isolated in the State of Minnesota (Galina et al., 1996). In the present study, we determined the prevalence of the different serotypes and carried out a genotypic and phenotypic assessment of the production of these virulence markers in S. suis isolates from the U.S.

Section snippets

S. suis isolates

A total of 100 S. suis field strains isolated in the U.S. between January, 2003 and December, 2005 were used in this study. The isolates had been submitted as part of routine diagnostic procedures to the University of Minnesota Veterinary Diagnostic Laboratory or the Iowa State University Veterinary Diagnostic Laboratory and identified as S. suis using standard biochemical tests. Criteria for inclusion in this study were as follows: (a) isolates were from different farms; (b) pigs must have

Serotyping

We first determined the serotype of the isolates included in the panel. Results showed that of the 100 isolates, serotype 3 (N = 20) was the most prevalent, followed by serotypes 2 (N = 17), 7 (N = 13), 8 (N = 11), 6 (N = 9) and 1/2 (N = 8). These 6 serotypes together accounted for 78% of the isolates. Four isolates belonged to serotype 5. Serotypes 1, 9 and 23 comprised three isolates each; serotype 18 two isolates, and serotypes 10, 12, 14, and 30, one isolate each. Finally, three strains were

Acknowledgements

We thank Karen Olsen (University of Minnesota Veterinary Diagnostic Laboratory) and Lorraine Hoffman (Iowa State University Veterinary Diagnostic Laboratory). We are grateful to C. Dozois (IAF-INRS, Laval, Canada) for critical reading of the manuscript. We thank Sonia Lacouture and Diane Coté for outstanding technical assistance. This work was financed by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC; 154280) to MG. NF is the recipient of an NSERC

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