Identification, purification, and characterization of a thiol-activated hemolysin (suilysin) of Streptococcus suis

The present report describes the identification, purification, and characterization of a hemolysin produced by Streptococcus suis type 2. The hemolysin was purified from the culture supernatant by using different filtration steps, Superose-12 column chromatography, and selective (NH4)2SO4 precipitation. The purified hemolysin, designated suilysin, had an apparent molecular mass of 54,000 Da and exhibited a specific activity of 0.7 x 10(6) hemolytic units per mg. Suilysin appeared to belong to a family of toxins known as the thiol-activated toxins, with which it had several characteristics in common: loss of activity upon oxidation, reactivation upon reduction, and inhibition of activity by small amounts of cholesterol. The N-terminal amino acid sequence of suilysin showed many similarities with parts of the deduced N-terminal amino acid sequences of perfringolysin O, streptolysin O, listeriolysin O, alveolysin, and pneumolysin. Mice immunized with a vaccine containing purified suilysin appeared to be completely protected against a lethal S. suis type 2 challenge, indicating that suilysin is an important factor and that the neutralization of this single factor is sufficient to protect mice against the detrimental effects of an S. suis type 2 infection. Most of the different (serotype) strains appeared to secrete hemolytic activity which was biochemically and immunologically indistinguishable from suilysin into the culture supernatant in vitro, indicating that suilysin might be a cross-protection factor.