Silibinin attenuates Streptococcus suis serotype 2 virulence by targeting suilysin.

To determine the antivirulence properties of silibinin against suilysin (SLY), a virulence factor of Streptococcus suis serotype 2 (SS2) that plays an important role in the pathogenesis of S. suis infection and its protective effect against SS2 infection in a mouse model. Susceptibility testing, haemolysis assay and Western blot assays were employed to evaluate the performance of silibinin on SLY pore-forming activity. Cytotoxicity assays and mouse infection tests were also performed to determine the efficacy of silibinin against SS2 infection. The results showed that silibinin, a flavonoid with little anti-S. suis activity, was identified to be a potent antagonist of SLY-mediated haemolysis through the inhibition of its oligomerization. Treatment with silibinin reduced S. suis-induced cytotoxicity in macrophages (J774 cells). In addition, S. suis-infected mice that received silibinin showed a lower bacterial burden. Our results demonstrated that silibinin is a promising candidate for the development of antivirulence therapeutic agents to treat S. suis infections. The antivirulent property of silibinin against SS2 by targeting SLY provides the possibility for the future pharmaceutical application of silibinin to prevent and treat S. suis infection.