Elsevier

The Veterinary Journal

Volume 209, March 2016, Pages 108-112
The Veterinary Journal

Screening of virulence-associated genes as a molecular typing method for characterization of Streptococcus suis isolates recovered from wild boars and pigs

https://doi.org/10.1016/j.tvjl.2015.11.007Get rights and content

Highlights

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    Virulence-associated genes were used to characterize S. suis from wild boars and pigs.

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    Wild boar isolates exhibited lower virulence genes detection rates than pig isolates.

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    Molecular typing suggests different populations among wild boars and pigs.

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    Virulence-associated gene typing could be useful for epidemiological purposes.

Abstract

Streptococcus suis is an important zoonotic pathogen associated with a wide range of diseases in pigs, but has also been isolated from wild animals such as rabbits and wild boars. In the current study, 126 S. suis isolates recovered from pigs (n = 85) and wild boars (n = 41) were tested by polymerase chain reaction (PCR) for the presence of nine virulence-associated genes. S. suis isolates from wild boars were differentiated by the lower detection rates of the epf, sly, mrp, sao and dltA genes (0%, 2.4%, 2.4%, 4.8% and 21.9%, respectively) compared with the isolates from pigs (56.5%, 75.3%, 56.5%, 88.2.0% and 88.2%, respectively). The differences in the content of these virulence-associated genes were statistically significant (P < 0.05). There was a correlation between the variants saoM and saoL and serotypes 2 and 9, respectively (P < 0.05). Isolates were classified into 31 virulence-associated gene profiles (VPs). Ten VPs were detected among wild boar isolates and 22 VPs among pig isolates, with only two VPs common to wild boars and pigs. The predominant VPs among isolates from wild boars (VP1, VP7) were different from those observed in pig isolates (VP16 and VP26). VP16 was detected exclusively in clinical pig isolates of serotype 9 and VP26 was detected in 71.4% of the serotype 2 clinical pig isolates. Further multilocus sequence typing (MLST) analysis showed a significant correlation association between certain VPs and STs (VP16 and VP17 with ST123 and ST125 and VP26 with ST1). In conclusion, the current study showed that combination of virulence-associated gene profiling and MLST analysis may provide more information of the relatedness of the S. suis strains from different animal species that could be useful for epidemiological purposes.

Introduction

Streptococcus suis is an important zoonotic pathogen associated with a wide range of diseases in pigs, including meningitis, septicemia and pneumonia (Gottschalk, 2012). S. suis causes important loses to the swine industry and is also recognized as an emerging human pathogen (Gottschalk et al., 2010). Furthermore, S. suis has been isolated from wild animals, such as rabbits and wild boars, that have been recognized as reservoirs of S. suis (Baums et al, 2007, Sánchez del Rey et al, 2013, Sánchez del Rey et al, 2014).

Subtyping of pathogen isolates is important for tracing new or existing clones. The diversity of S. suis populations has been studied using different methods, such as serotyping, ribotyping (Staats et al, 1998, Rasmussen et al, 1999), random amplified polymorphic DNA (RADP) (Martinez et al, 2002, Cloutier et al, 2003), pulsed field gel electrophoresis (PFGE) (Vela et al, 2003, Blume et al, 2009, Luque et al, 2010), multilocus sequence typing (MLST) (King et al, 2002, Blume et al, 2009) or multiple loci VNTR analysis (MLVA) (Li et al, 2010, Sánchez del Rey et al, 2014). In addition to these typing techniques, virulence-associated gene profiling is a useful tool for the molecular characterization of bacterial pathogens for epidemiological purposes (Nowrouzian et al, 2013, Schierack et al, 2013).

The majority of studies on S. suis have investigated only the presence of the genes sly, epf and mrp, encoding the virulence-associated markers suilysin (SLY), extracellular factor (EF) and muramidase-released protein (MRP), respectively. These studies have included human and/or swine isolates (Wisselink et al, 2000, Rehm et al, 2007, Blume et al, 2009, Fittipaldi et al, 2009, Schultsz et al, 2012). Recently, virulence-associated gene profiling using a set of genes other than epf, sly and mrp has been successfully used for the characterization of S. suis isolates from wild rabbits (Sánchez del Rey et al., 2013). However, there is no information available regarding the distribution of these markers in the swine population.

In the present study, virulence-associated gene profiling was used to characterize a set of S. suis isolates from pigs and wild boars, and their virulence genes profiles were further compared with the genotypes observed in wild rabbits (Sánchez del Rey et al., 2013). We also examined the correlation between sequence types (STs) and virulence-associated gene profiles in S. suis to evaluate the utility of virulence gene determination as a complementary tool for epidemiological typing of S. suis.

Section snippets

Streptococcus suis isolates

In this study, we investigated the virulence-associated gene profiles of 126 S. suis isolates recovered from swine (n = 85) and wild boar (n = 41). Swine isolates included 29 tonsillar (serotype 2, n = 20 and serotype 9, n = 9) and 56 clinical (serotype 2, n = 28 and serotype 9, n = 28) isolates. Except for one clinical isolate of serotype 2, tonsillar and clinical wild boar isolates were all of serotype 9 (n = 36 and n = 4, respectively).

Presence of virulence-associated genes

All isolates were tested for the presence of nine

Results

The detection rates of the virulence-associated genes in wild boar and pigs S. suis isolates are shown in Table 1. Most S. suis isolates from wild boars were differentiated by the low detection rates of the epf, sly, mrp, sao and dltA genes (0%, 2.4%, 2.4%, 4.8% and 21.9%, respectively). These genes were detected in at least half of the S. suis pig isolates (56.5%, 75.3%, 56.5%, 88.2% and 88.2%, respectively). The differences in the content of these virulence associated genes between S. suis

Discussion

S. suis is one of the most important swine pathogens worldwide and is considered a major problem in the swine industry (Gottschalk, 2012). Serotype 2 is the most prevalent serotype associated with disease in pigs (Wisselink et al, 2000, Gottschalk et al, 2007, Wei et al, 2009), but other serotypes, such as serotype 9, have gained relevance in the last years (Wisselink et al, 2000, Silva et al, 2006, Blume et al, 2009, Schultsz et al, 2012). S. suis, particularly isolates of serotype 9, have

Conclusions

There are differences in the occurrence and distribution of virulence-associated genes between S. suis isolates from some wild animals species and pigs. Virulence-associated gene profiling is a low cost, easy to perform method that could be a useful method for epidemiological purposes and that could contribute to a better understanding of the global epidemiology of S. suis.

Conflict of interest statement

None of the authors of this paper has a financial or personal relationship with other people or organizations that could inappropriately influence or bias the content of the paper.

Acknowledgements

This work was supported by the project AGL2009-14303-C02-01 of the Spanish Ministry of Education and Science (MEC). Verónica Sánchez del Rey is the recipient of a PhD grant from the Spanish MEC.

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