Surface-associated and secreted factors of Streptococcus suis in epidemiology, pathogenesis and vaccine development

Streptococcus suis is an invasive porcine pathogen associated with meningitis, arthritis, bronchopneumonia and other diseases. The pathogen constitutes a major health problem in the swine industry worldwide. Furthermore, S. suis is an important zoonotic agent causing meningitis and other diseases in humans exposed to pigs or pork. Current knowledge on pathogenesis is limited, despite the enormous amount of data generated by 'omics' research. Accordingly, immunoprophylaxis (in pigs) is hampered by lack of a cross-protective vaccine against virulent strains of this diverse species. This review focuses on bacterial factors, both surface-associated and secreted ones, which are considered to contribute to S. suis interaction(s) with host factors and cells. Factors are presented with respect to (i) their identification and features, (ii) their distribution among S. suis and (iii) their significance for virulence, immune response and vaccination. This review also shows the enormous progress made in research on S. suis over the last few years, and it emphasizes the numerous challenging questions remaining to be answered in the future.