Suilysin production by Streptococcus suis strains isolated from diseased and healthy carrier pigs in Spain

Genetic analysis of Streptococcus suis isolates recovered from diseased and healthy carrier pigs at different stages of production on a pig farm

Streptococcus suis isolates from pigs at different stages of production on a farrow-to-finish farm were characterised by serotyping, pulsed-field gel electrophoresis (PFGE) and production of muramidase-released protein, extracellular factor and suilysin. S. suis was isolated from the tonsils of 81/287 (28.2%) healthy pigs: 16/47 (34%) post-weaning, 18/47 (38.3%) transition, 18/47 (38.3%) fattening and 29/146 (19.9%) sows. A total of 127 S. suis isolates were analysed: 14 from diseased pigs at the post-weaning stage and 113 from the tonsils of healthy pigs. Serotypes 2, 4, 9, 14 and 1/14 were isolated from both diseased and healthy pigs. A total of 83 PFGE profiles were obtained; most isolates (95.2%) were grouped into three clusters (A-C). Animals at different production stages harboured isolates with similar phenotypic and genetic profiles, highlighting the importance of healthy animals in the maintenance of strains responsible for outbreaks of clinical disease.

SalK/SalR, a two-component signal transduction system, is essential for full virulence of highly invasive Streptococcus suis serotype 2

Background

Streptococcus suis serotype 2 (S. suis 2, SS2) has evolved into a highly infectious entity, which caused the two recent large-scale outbreaks of human SS2 epidemic in China, and is characterized by a toxic shock-like syndrome. However, the molecular pathogenesis of this new emerging pathogen is still poorly understood.

Methodology/Principal Findings

89K is a newly predicted pathogenicity island (PAI) which is specific to Chinese epidemic strains isolated from these two SS2 outbreaks. Further bioinformatics analysis revealed a unique two-component signal transduction system (TCSTS) located in the candidate 89K PAI, which is orthologous to the SalK/SalR regulatory system of Streptococcus salivarius. Knockout of salKR eliminated the lethality of SS2 in experimental infection of piglets. Functional complementation of salKR into the isogenic mutant ΔsalKR restored its soaring pathogenicity. Colonization experiments showed that the ΔsalKR mutant could not colonize any susceptible tissue of piglets when administered alone. Bactericidal assays demonstrated that resistance of the mutant to polymorphonuclear leukocyte (PMN)-mediated killing was greatly decreased. Expression microarray analysis exhibited a transcription profile alteration of 26 various genes down-regulated in the ΔsalKR mutant.

Conclusions/Significance

These findings suggest that SalK/SalR is requisite for the full virulence of ethnic Chinese isolates of highly pathogenic SS2, thus providing experimental evidence for the validity of this bioinformatically predicted PAI.

Streptococcus suis infections in humans: the Chinese experience and the situation in North America

Infections caused by Streptococcus suis are considered a global problem in the swine industry. In this animal species, S. suis is associated with septicemia, meningitis, endocarditis, arthritis and, occasionally, other infections. Moreover, it is an agent of zoonosis that afflicts people in close contact with infected pigs or pork-derived products. Although sporadic cases of S. suis infection in humans have been reported, a large outbreak due to S. suis serotype 2 emerged in the summer of 2005 in Sichuan, China. A similar outbreak was observed in another Chinese province in 1998. Symptoms reported in these two outbreaks include high fever, malaise, nausea and vomiting, followed by nervous symptoms, subcutaneous hemorrhage, septic shock and coma in severe cases. The increased severity of S. suis infections in humans, such as a shorter incubation time, more rapid disease progression and higher rate of mortality, underscores the critical need to better understand the factors associated with pathogenesis of S. suis infection. From the 35 capsular serotypes currently known, serotype 2 is considered the most virulent and frequently isolated in both swine and humans. Here, we review the epidemiological, clinical and immunopathological features of S. suis infection in humans.

Development of a multilocus sequence typing scheme for the pig pathogen Streptococcus suis: identification of virulent clones and potential capsular serotype exchange

Streptococcus suis is an important pathogen of pigs and occasionally causes serious human disease. However, little is known about the S. suis population structure, the clonal relationships between strains, the potential of particular clones to cause disease, and the relevance of serotype as a marker for epidemiology. Here we describe a multilocus sequence typing (MLST) scheme for S. suis developed in order to begin to address these issues. Seven housekeeping gene fragments from each of 294 S. suis isolates obtained from various S. suis diseases and from asymptomatic carriage representing 28 serotypes and nine distinct countries of origin were sequenced. Between 32 and 46 alleles per locus were identified, giving the ability to distinguish >1.6 x 10(11) sequence types (STs). However only 92 STs were identified in this study. Of the 92 STs 18 contained multiple isolates, the most common of which, ST1, was identified on 141 occasions from six countries. Assignment of the STs to lineages resulted in 37 being identified as unique and unrelated STs while the remaining 55 were assigned to 10 complexes. ST complexes ST1, ST27, and ST87 dominate the population; while the ST1 complex was strongly associated with isolates from septicemia, meningitis, and arthritis, the ST87 and ST27 complexes were found to contain significantly higher numbers of lung isolates. In agreement with the observed distribution of disease-causing isolates of S. suis, most isolates previously characterized as of high virulence in porcine infection models belong to ST1, while isolates belonging to other STs appear to be less virulent in general. Finally nine STs were found to contain isolates of multiple serotypes, and many isolates belonging to the same serotypes were found to have very disparate genetic backgrounds. As well as highlighting that the serotype can often be a poor indicator of genetic relatedness between S. suis isolates, these findings suggest that capsular genes may be moving horizontally through the S. suis population.

Evidence for lateral transfer of the Suilysin gene region of Streptococcus suis

Suilysin is a cholesterol-binding cytolysin encoded by sly in Streptococcus suis. DNA sequence determination of the sly locus in a strain lacking sly revealed the presence of another gene, designated orf102, in the place of sly. No transposable element or long-repeat sequence was found in the close vicinity. Except for six strains whose corresponding loci have been rearranged, all of the remaining 62 strains examined had either sly or orf102 at the same locus and their flanking regions were conserved. The genetic organizations having either sly or orf102 were found in the strains whose 16S rRNA sequences were identical. These results suggest that S. suis acquired sly or orf102 from a foreign source and that these genes subsequently spread among S. suis strains by homologous recombination.

Distribution and genetic diversity of suilysin in Streptococcus suis isolated from different diseases of pigs and characterization of the genetic basis of suilysin absence

Streptococcus suis is an economically important pathogen of pigs responsible for a variety of diseases including meningitis, septicemia, arthritis, and pneumonia, although little is known about the mechanisms of pathogenesis or virulence factors associated with this organism. Here, we report on the distribution and genetic diversity of the putative virulence factor suilysin, a member of the thiol-activated toxin family of gram-positive bacteria. On the basis of PCR analysis of over 300 isolates of S. suis, the suilysin-encoding gene, sly, was detected in 69.4% of isolates. However, sly was present in a considerably higher proportion of isolates obtained from cases of meningitis, septicemia, and arthritis (>80%) and isolates obtained from asymptomatic tonsillar carriage (>90%) than lung isolates associated with pneumonia (44%). With the exception of serotypes 1, 14, and 1/14, there was no strong correlation between the presence of suilysin and serotype. Analysis of the genetic diversity of suilysin by restriction fragment length polymorphism and sequence analysis found that the suilysin gene, where present, is highly conserved with a maximum of 1.79% diversity at the nucleotide level seen between sly alleles. Assays of hemolytic activity and hybridization analysis provided no evidence for a second member of the thiol-activated toxin family in S. suis. Inverse PCR was used to characterize regions flanking sly, which in turn allowed the first characterization of the equivalent region in a strain lacking sly. Sequence comparison of these regions from sly-positive (P1/7) and sly-negative (DH5) strains indicated that two alternative arrangements are both flanked by genes with highest similarity to haloacid dehalogenase-like hydrolases (5' end) and putative N-acetylmannosamine-6-phosphate epimerases (3' end). However, sly appears to be completely absent from the alternative arrangement, and a gene of unknown function is located in the equivalent position. Finally, PCR analysis of multiple sly-positive and -negative strains indicated that these two alternative genetic arrangements are conserved among many S. suis isolates.