Selection of virulence-associated determinants of Streptococcus suis serotype 2 by in vivo complementation

Within Streptococcus suis serotype 2, pathogenic, weakly pathogenic, and no npathogenic strains can he found. We introduced a genomic library of a path ogenic strain into a weakly pathogenic strain. After infection of the libra ry into young piglets pathogenic transformants were selected. One specific transformant containing a 3-kb fragment of the pathogenic strain appeared t o 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 conside rably. 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 pre sence of two potential open reading frames, both of which were found to be mutated in the corresponding fragment of the weakly pathogenic strain. Thes e data strongly suggest that the selected fragment contains determinants im portant for virulence.