Streptococcus suis serotype 2 (S. suis 2), an important zoonotic pathogen worldwide, has evolved to be a serious problem over the past two decades [1–3]. It was reported that S. suis 2 only causes sporadic cases of human infection with a mortality of less than 10% [4, 5]. However, it emerged as the leading cause of two large-scale outbreaks of severe epidemics in China in 1998 and 2005, respectively . The unusual outbreaks affected over 200 individuals and killed 52 of them [7, 8]. Besides its large size and the associated high mortality rate, these two outbreaks are unique in that a large proportion of patients were victim to streptococcal toxic shock syndrome (STSS) . Before that, STSS has been limited to disease caused by the group A streptococcus , S. suis (nongroup A) has not previously been linked to STSS.
To get insight into the high virulence of the S. suis isolates emerged in China, we previously decoded the whole genomic sequence of two epidemic strains (98HAH12 and 05ZYH33) isolated from the 1998 and 2005 Chinese outbreaks respectively, and identified a pathogenicity island (PAI) designated 89K that is specific for Chinese outbreak isolates [10, 11]. Subsequently, we provided genetic evidence showing that an 89K-borne type IV secretion system (T4SS) forms an important pathway for horizontal transfer of 89K and secretion of some unknown pathogenic effectors that are responsible for STSS caused by the highly virulent S. suis 2 strains [12, 13]. However, the 89K T4SS assembly process in vivo and in vitro remains largely unknown.
There has long been a general lack of knowledge of T4SS functions and cellular localization in gram-positive bacteria . It has been suggested that the assembly processes must be similar to or even simpler than those in gram-negative bacteria [15, 16]. In the well-characterized model for the Agrobacterium tumefaciens VirB/D T4SS, the VirB1 component functions as a lytic transglycosylase that can digest the peptidoglycan layer of cell wall, thus facilitating the assembly of envelope-spanning protein complex of T4SS under temporal and spatial control [17, 18]. Among the single operon composed of 15 genes that encodes the functional T4SS in S. suis 89K PAI, only the virB1-89K gene product shows similarity to the Agrobacterium VirB1 component and contains a conserved cysteine, histidine-dependent amidohydrolases/peptidases (CHAP) domain that may function in peptidoglycan hydrolysis . We once proposed that VirB1-89K should function to punch holes in the peptidoglycan cell wall to allow the assembly of the T4SS apparatus . However, we did not provide direct evidence to support this hypothesis.
In the present study, therefore, we expressed and purified the CHAP domain of VirB1-89K in Escherichia coli, and tested its putative peptidoglycan hydrolysis activity in vitro. Furthermore, an isogenic knockout mutant of virB1-89K and its complementary strain were used in a mouse infection model to assess the contribution of VirB1-89K to the virulence of S. suis outbreak strain. The experimental results indicated that VirB1-89K facilitates the assembly of 89K T4SS apparatus by catalyzing the degradation of the peptidoglycan cell wall, thus contributing to the pathogenesis of T4SS in the S. suis.