Streptococcus suis infection: An emerging/reemerging challenge of bacterial infectious diseases?

Feng, Youjun; Zhang, Huimin; Wu, Zuowei; Wang, Shihua; Cao, Min; Hu, Dan; Wang, Changjun

The serum opacity factor of S. suis (OFS) is a putative member belonging to the family of MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules). 120 N-terminal region of OFS exhibits similarity to the serum opacity factor of Streptococcus pyogenes and fibronectin-binding protein A (FnBA) of Streptococcus dysgalactiae, and its C-terminus harbors repetitive sequence elements. Crude extract of S. suis and the recombinant OFS protein both possessed serum opacification activity. Experimental infections demonstrated that the deletion of the ofs gene severely impairs S. suis virulence. 120 Sao is a surface antigen first identified by Li et al., 121 which can react with convalescent-phase sera from pigs clinically infected by S. suis type 2. Subsequently recombinant Sao formulated with Quil A was found to induce potent opsonizing antibody responses, and confer cross-protection of mice against challenges of virulent heterogeneous S. suis. 122 We also discovered that three allelic variants of the sao gene (namely sao-S, sao-M, and sao-L) are present in S. suis population, and we have developed an efficient ELISA method in which Sao-M protein serves as the capture antigen. 36 Recent further genetic study suggested that SAO protein is only a minor virulence factor 123 Sat (HP272) is another newly-identified surface protein from the S. suis serotype 2, 111,112 and two different research groups have shown evidence that recombinant Sat protein can confer effective immuno-protection of mice against SS2 infections, implying that it is a vaccine molecule candidate. 111, 112 In addition, HtpS, a putative member of histidine triad protein family, was determined to be cell surface-associated protein that was expressed during the infection of Chinese SS2 strain 05ZYH33. 110 Moreover, recombinant HtpS protein was demonstrated to function as a protective antigen against SS2 infections. 110 Similarly, Zhang et al. 124 recently defined another new infection-associated antigen protein, Trag, using in vivo-induced antigen technology (IVIAT). An inactivation of the trag gene was observed to attenuate full virulence of Chinese SS2 strain in the experimental model of Zebrafish. 124 As a surface protein, PAPI-2 was found to constitute an ancillary pilus subunit and exhibit ability to confer protection of mice against serious challenge of S. suis. 125 Although we have never reported evidence that Zur, a zinc uptake regulator is essential for S. suis virulence, 29 Aranda et al. very recently reported that the zinc-binding lipoprotein 103, the structural component of zinc uptake system is associated with the infectivity of Streptococcus suis, implying that zinc uptake system might be involved into bacterial pathogenesis. 126 This discrepancy may be attributed to the fact that the deletion of zur only partially affect expression of zinc uptake system genes, whereas inactivation of "103" completely impairs zinc uptake system. That is why the latter could more seriously disrupt the normal zinc metabolism, and in turn lead to virulence attenuation in this sick bacterium.