Abstract
The capsular polysaccharide (CPS) represents a key virulence factor for most encapsulated streptococci. Streptococcus suis and Group B Streptococcus (GBS) are both well-encapsulated pathogens of clinical importance in veterinary and/or human medicine and responsible for invasive systemic diseases. S. suis and GBS are the only Gram-positive bacteria which express a sialylated CPS at their surface. An important difference between these two sialylated CPSs is the linkage between the side-chain terminal galactose and sialic acid, being α-2,6 for S. suis but α-2,3 for GBS. It is still unclear how sialic acid may affect CPS production and, consequently, the pathogenesis of the disease caused by these two bacterial pathogens. Here, we investigated the role of sialic acid and the putative effect of sialic acid linkage modification in CPS synthesis using inter-species allelic exchange mutagenesis. To this aim, a new molecular biogenetic approach to express CPS with modified sialic acid linkage was developed. We showed that sialic acid (and its α-2,6 linkage) is crucial for S. suis CPS synthesis, whereas for GBS, CPS synthesis may occur in presence of an α-2,6 sialyltransferase or in absence of sialic acid moiety. To evaluate the effect of the CPS composition/structure on sialyltransferase activity, two distinct capsular serotypes within each bacterial species were compared (S. suis serotypes 2 and 14 and GBS serotypes III and V). It was demonstrated that the observed differences in sialyltransferase activity and specificity between S. suis and GBS were serotype unrestricted. This is the first time that a study investigates the interspecies exchange of capsular sialyltransferase genes in Gram-positive bacteria. The obtained mutants represent novel tools that could be used to further investigate the immunomodulatory properties of sialylated CPSs. Finally, in spite of common CPS structural characteristics and similarities in the cps loci, sialic acid exerts differential control of CPS expression by S. suis and GBS.
Highlights
Capsular polysaccharides (CPSs) play critical roles in the pathogenesis of the disease caused by several bacterial pathogens, including streptococci
To confirm that the cps locus of S. suis is encoding a single polycistronic transcript and to confirm that the sialyltransferase is under the same promoter than other CPS synthesis genes, RNA obtained from an overnight culture of S. suis was analyzed by RT-PCR
Using appropriate primers from adjacent genes (Supplementary Table S1), we showed that all genes within the cps coding locus are transcripted as a single polycistronic transcript (Figure 3)
Summary
Capsular polysaccharides (CPSs) play critical roles in the pathogenesis of the disease caused by several bacterial pathogens, including streptococci. Of the initially described 35 capsular types or serotypes, S. suis type 2 predominates worldwide in both pigs and humans Besides this important and highly virulent serotype, type 14 is emerging as a threat to human health (Goyette-Desjardins et al, 2014). GBS is an important cause of severe invasive bacterial infections in humans worldwide (Johri et al, 2006; Madzivhandila et al, 2011). GBS is classified into 10 different serotypes, and type III is the most common type in GBS meningitis, whereas serotype V has long been recognized as a leading cause of invasive disease in adults (Johri et al, 2006; Madzivhandila et al, 2011)
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