Comparative Molecular Modelling of Capsular Polysaccharide Conformations in Streptococcus suis Serotypes 1, 2, 1/2 and 14 Identifies Common Epitopes for Antibody Binding

Streptococcus suis serotypes 2, 1/2, 1, and 14 isolated from the brain of pigs dead with neurological symptoms or sudden death in Italian farms

The Streptococcus suis serotypes 2, and 14 are zoonotic pathogens in swine and present a capsular polysaccharide structure closely related to serotypes 1 and 1/2. Most molecular methods applied in veterinary diagnostic laboratories do not detect the single nucleotide polymorphism (SNP) that differentiates the serotypes 2 from 1/2, and 1 from 14. Consequently, epidemiological data is lacking. A collection of strains isolated between 2019 and 2024 from the brain of diseased pigs, and previously classified as 2-1/2 or 1-14, was analysed using three molecular methods able to detect of the SNP distinguishing the four serotypes. The study revealed that 57 % of those formerly classified as serotype 2-1/2 actually belong to serotype 1/2, while no serotype 14 was found among the strains classified as 1-14. The study also evaluates the feasibility of the routine application of these PCR-based methods for SNP detection, emphasizing the importance for accurate and rapid serotype identification to aid vaccine production and epidemiology.

Identification of SepF in Streptococcus suis involving cell division

BACKGROUND

Streptococcus suis (S. suis) is a major zoonotic pathogen that infects humans and pigs. The increasing emergence and dissemination of antibiotic resistance bacteria accelerates the urgent need to develop novel drug targets. Bacterial cell divisome is attractive target. FtsZ, an essential tubulin-like protein, forms a Z-ring that executes the synthesis of the divisome. However, the exact division process of S. suis remains unknown.

RESULTS

here, we reported a SepF homolog from S. suis that modulated the function of FtsZ. sepF disruption was not lethal and its deletion mutant (∆sepF) displayed normal growth rate. ∆sepF exhibited long chains, occasionally anuclear daughter cells. Electron microscope revealed that the lack of SepF in cells led to abnormal septum which twisted out of shape, and disturbed cell division due to an increased length-width ratio and multiple septal peptidoglycan wall in a cell compared to the wild type strain. Mechanistic studies showed that SepF interacted with FtsZ to promote the bundling of FtsZ protofilaments. Furthermore, sub-cellular localization of FtsZ-GFP in ∆sepF also confirmed the abnormal septum and cell morphology.

CONCLUSIONS

These results showed that SepF was a cell division protein in S. suis responsible for maintaining cell shape and regulating FtsZ localization.

AI-2 quorum sensing-induced galactose metabolism activation in Streptococcus suis enhances capsular polysaccharide-associated virulence

Bacteria utilize intercellular communication to orchestrate essential cellular processes, adapt to environmental changes, develop antibiotic tolerance, and enhance virulence. This communication, known as quorum sensing (QS), is mediated by the exchange of small signalling molecules called autoinducers. AI-2 QS, regulated by the metabolic enzyme LuxS (S-ribosylhomocysteine lyase), acts as a universal intercellular communication mechanism across gram-positive and gram-negative bacteria and is crucial for diverse bacterial processes. In this study, we demonstrated that in Streptococcus suis (S. suis), a notable zoonotic pathogen, AI-2 QS enhances galactose utilization, upregulates the Leloir pathway for capsular polysaccharide (CPS) precursor production, and boosts CPS synthesis, leading to increased resistance to macrophage phagocytosis. Additionally, our molecular docking and dynamics simulations suggest that, similar to S. pneumoniae, FruA, a fructose-specific phosphoenolpyruvate phosphotransferase system prevalent in gram-positive pathogens, may also function as an AI-2 membrane surface receptor in S. suis. In conclusion, our study demonstrated the significance of AI-2 in the synthesis of galactose metabolism-dependent CPS in S. suis. Additionally, we conducted a preliminary analysis of the potential role of FruA as a membrane surface receptor for S. suis AI-2.

Modeling of pneumococcal serogroup 10 capsular polysaccharide molecular conformations provides insight into epitopes and observed cross-reactivity

Streptococcus pneumoniae is an encapsulated gram-negative bacterium and a significant human pathogen. The capsular polysaccharide (CPS) is essential for virulence and a target antigen for vaccines. Although widespread introduction of pneumococcal conjugate vaccines (PCVs) has significantly reduced disease, the prevalence of non-vaccine serotypes has increased. On the basis of the CPS, S. pneumoniae serogroup 10 comprises four main serotypes 10A, 10B, 10C, and 10F; as well as the recently identified 10D. As it is the most prevalent, serotype 10A CPS has been included as a vaccine antigen in the next generation PCVs. Here we use molecular modeling to provide conformational rationales for the complex cross-reactivity reported between serotypes 10A, 10B, 10C, and 10F anti-sera. Although the highly mobile phosphodiester linkages produce very flexible CPS, shorter segments are conformationally defined, with exposed β-D-galactofuranose (β DGalf) side chains that are potential antibody binding sites. We identify four distinct conformational epitopes for the immunodominant β DGalf that assist in rationalizing the complex asymmetric cross-reactivity relationships. In particular, we find that strongly cross-reactive serotypes share common epitopes. Further, we show that human intelectin-1 has the potential to bind the exposed exocyclic 1,2-diol of the terminal β DGalf in each serotype; the relative accessibility of three- or six-linked β DGalf may play a role in the strength of the innate immune response and hence serotype disease prevalence. In conclusion, our modeling study and relevant serological studies support the inclusion of serotype 10A in a vaccine to best protect against serogroup 10 disease.