Relatedness of Streptococcus suis Isolates of Various Serotypes and Clinical Backgrounds as Evaluated by Macrorestriction Analysis and Expression of Potential Virulence Traits

Abstract

We evaluated the genetic diversity of Streptococcus suis isolates of different serotypes by macrorestriction analysis and elucidated possible relationships between the genetic background, expression of potential virulence traits, and source of isolation. Virulence traits included expression of serotype-specific polysaccharides, muramidase-released protein (MRP), extracellular protein factor (EF), hemolysin activity, and adherence to epithelial cells. Macrorestriction analysis of streptococcal DNA digested with restriction enzymes SmaI and ApaI allowed differentiation of single isolates that could be assigned to four major clusters, named A1, A2, B1, and B2. Comparison of the genotypic and phenotypic features of the isolates with their source of isolation showed that (i) the S. suis population examined, which originated mainly from German pigs, exhibited a genetic diversity and phenotypic patterns comparable to those found for isolates from other European countries; (ii) certain phenotypic features, such as the presence of capsular antigens of serotypes 2, 1, and 9, expression of MRP and EF, and hemolysin activity (and in particular, combinations of these features), were strongly associated with the clinical background of meningitis and septicemia; and (iii) isolates from pigs with meningitis and septicemia showed a significantly higher degree of genetic homogeneity compared to that for isolates from pigs with pneumonia and healthy pigs. Since the former isolates are considered highly virulent, this supports the theory of a clonal relationship among highly virulent strains.Streptococcus suis is a major cause of meningitis, septicemia, arthritis, and bronchopneumonia in young pigs and can cause meningitis in humans (1, 2). Effective control of the disease is hampered by the poor knowledge about its epidemiology and pathogenesis. Since many healthy pigs harbor S. suis as an "early colonizer" (6, 18), identification of virulent S. suis isolates is of particular importance. This is, however, complicated by the pathogen's extreme diversity, in particular with respect to its virulence.Phenotypic markers used to distinguish highly virulent and avirulent isolates include the presence of serotype-specific capsular polysaccharides (25), expression of muramidase-relased protein (MRP) (27, 35) and extracellular protein factor (EF) (35), and hemolysin activity (10,11,16). The role of these factors as virulence markers, however, is still unclear. The capsular polysaccharides are the basis for classification into serotypes (of which 35 are currently known) and have recently been shown to prevent phagocytosis and, thus, have been proposed as an important virulence trait (25). Serotype 2 is considered the most dominant one among highly virulent strains, but disease is also frequently caused by strains of other serotypes (24, 33, 36), suggesting that serotype-independent virulence markers must exist. MRP and EF have previously been found to be strongly associated with highly virulent isolates in Europe (35, 36) but do not appear t...

Analysis of Genetic Diversity of Streptococcus suis Clinical Isolates from Pigs in Spain by Pulsed-Field Gel Electrophoresis

“…suis can be differentiated on the basis of distinct polysaccharide capsular antigens. In this study, the results of serotyping were in agreement with those of other studies (1,2,5,15,20,26,30) in that capsular types 2 and 9 are the most prevalent in many European countries, including Spain. However, the frequency of serotype 2 isolation was about 50% lower than usual in Spain while serotype 9 was isolated in a much higher proportion (15,20,26,30).…”

“…As expected, the discriminatory power of this technique improved when both enzymes were combined (29). The great diversity observed in this study agrees with previous reports (2,7,18,22,24,25) and could explain the differences in virulence observed not only between serotypes but also between S. suis strains of the same serotype (24).…”

“…Most of the molecular characterization studies of S. suis have been focused on the comparison between isolates from diseased or healthy pigs or between human and swine isolates (2,7,8). In the present study, PFGE was used for the genetic characterization of a total of 302 S. suis clinical isolates of various serotypes isolated from 74 herds.…”

“…Among the 34 different capsular types actually described, not all serotypes have similar clinical relevance or are equally important in the different countries. Serotype 2 is the most frequently isolated worldwide, including many European countries (2,26,30). Serotype 9 is most prevalent in Australia, Germany, Belgium, and The Netherlands (30), whereas in the United Kingdom, the most prevalent serotypes are 1 and 14 (1,3,12).…”

“…Pulsed-field gel electrophoresis (PFGE) is probably one of the most powerful molecular typing methods and has been successfully applied to different bacterial pathogens (4). Recently, PFGE has been used to study the genetic diversity of German and French S. suis swine isolates (2,7). Infections by S. suis represent one of the most important sanitary and economic problems in the Spanish intensive swine industry (19).…”