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 sids is an economically important pathogen of pigs responsibl e for a variety of diseases including meningitis, septicemia, arthritis, an d 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 suilysi n, 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 pre sent in a considerably higher proportion of isolates obtained from cases of meningitis, septicemia, and arthritis (> 80%) and isolates obtained from a symptomatic tonsillar carriage (> 90%) than lung isolates associated with p neumonia (44%). With the exception of serotypes 1, 14, and 1/14, there was no strong correlation between the presence of suilysin and serotype. Analys is of the genetic diversity of suilysin by restriction fragment length poly morphism and sequence analysis found that the suilysin gene, where present, is highly conserved with a maximum of 1.79% diversity at the nucleotide le vel seen between sly alleles. Assays of hemolytic activity and hybridizatio n analysis provided no evidence for a second member of the thiol-activated toxin family in S. suis. Inverse PCR was used to characterize regions flank ing 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 (DHS) strains indicated that two alter native arrangements are both flanked by genes with highest similarity to ha loacid dehalogenase-like hydrolases (S' end) and putative N-acetylmannosami ne-6-phosphate epimerases (3' end). However, sly appears to be completely a bsent from the alternative arrangement, and a gene of unknown function is l ocated in the equivalent position. Finally, PCR analysis of multiple sly-po sitive and -negative strains indicated that these two alternative genetic a rrangements are conserved among many S. suis isolates.