Within Streptococcus suis serotype 2, pathogenic, weakly pathogenic, and nonpathogenic strains can be found. We introduced a genomic library of a pathogenic strain into a weakly pathogenic strain. After infection of the library into young piglets pathogenic transformants were selected. One specific transformant containing a 3-kb fragment of the pathogenic strain appeared to be dominantly enriched in diseased pigs. The observed enrichment was not tissue specific. The selected fragment, when introduced into two different weakly pathogenic strains, increased the virulence of these strains considerably. In contrast, introduction of the corresponding fragment of a weakly pathogenic strain had only minor effects on virulence. Nucleotide sequence analysis of the selected fragment of the pathogenic strain revealed the presence of two potential open reading frames, both of which were found to be mutated in the corresponding fragment of the weakly pathogenic strain. These data strongly suggest that the selected fragment contains determinants important for virulence.Streptococcus suis is an important cause of meningitis, septicemia, arthritis, and sudden death in young pigs (6,32). It can also cause meningitis in humans (1). Attempts to control the disease are still hampered by the lack of sufficient knowledge about the pathogenesis of the disease and the lack of effective vaccines and sensitive diagnostic methods.So far, 35 serotypes of S. suis have been described (8-10). Virulence of S. suis can differ within and among serotypes (30,31,33,34). Worldwide, S. suis serotype 2 is the most frequently isolated serotype. Within S. suis serotype 2, pathogenic, weakly pathogenic, and nonpathogenic strains can be found (33, 34). The pathogenic strains cause severe clinical signs of disease in pigs, and large numbers of bacteria can be reisolated from the central nervous system (CNS) and the joints after experimental infection (33, 34). The weakly pathogenic strains cause only mild clinical signs of disease, and only infrequently can bacteria be reisolated from the CNS and the joints after experimental infection (33, 34). The nonpathogenic strains are completely avirulent in young pigs after experimental infection (33, 34).The 136-kDa muramidase-related protein and the 110-kDa extracellular factor are generally considered important virulence markers for S. suis serotype 2 strains isolated in Europe and the United States (2,7,17,22,29,36). However, differences in virulence between pathogenic, weakly pathogenic, and nonpathogenic strains cannot be explained exclusively by differences in their muramidase-related protein and extracellular factor expression patterns (27). In addition, it is known that the capsule of S. suis serotype 2 is an important virulence factor (24). However, since both pathogenic, weakly pathogenic, and nonpathogenic strains seem to be fully encapsulated after growth in vitro and in vivo (H. E. Smith and H. J. Wisselink, unpublished data), it is not likely that the level of encapsulation of these strains is assoc...

The identification of environmentally regulated genes of Streptococcus suis by the use of iron-restricted conditions in vitro and by experimental infection of piglets is described. Eighteen unique iron-restriction-induced (iri) genes and 22 unique in-vivo-selected (ivs) genes of Strep. suis were found. None of the ivs genes was exclusively expressed in vivo. Four iri genes were identical to four clones selected in piglets. Two ivs genes were similar to genes for putative virulence factors. One of these ivs genes was identical to the epf gene of virulent Strep. suis serotype 2 strains and the other showed homology to a gene encoding a fibronectin-binding protein of Streptococcus gordonii. Two additional ivs genes showed homology to environmentally regulated genes previously identified by using an in vivo expression technology (IVET) selection system in other bacterial species. One of these showed similarity to the agrA gene of Staphylococcus aureus, a key locus involved in the regulation of numerous virulence proteins. The promoter selection system described in this paper has been successfully used for the identification of many environmentally regulated genes potentially involved in the pathogenesis of Strep. suis infections in piglets.

Recent epidemiological studies on Streptococcus suis infections in pigs indicated that, besides serotypes 1, 2 and 9, serotype 7 is also frequently associated with diseased animals. For the latter serotype, however, no rapid and sensitive diagnostic methods are available. This hampers prevention and control programs. Here, we describe the development of a type-specific PCR test for the rapid and sensitive detection of S. suis serotype 7. The test is based on DNA sequences of capsular (cps) genes specific for serotype 7. These sequences were identified by cross-hybridization of several individual cps genes with the chromosomal DNAs of 35 different S. suis serotypes.

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