Innate immune response to Streptococcus suis infection: advances and future challenges

Streptococcus suis is a pathogenic bacterium primarily found in pigs, where it causes sudden death and meningitis, but it is also capable of causing septic shock and meningitis in humans, being the most important cause of adult bacterial meningitis in Southeast Asia. As the host's innate immune system serves as the first line of defense against microbial invaders, it is crucial to understand the intricate relationship between the bacterium and this system. Despite an increasing number of studies performed in the last 20 years, the pathogenesis of S. suis infection is not fully understood, and there is still an absence of effective vaccines on the market. Recent population-genomic studies have further highlighted that pathogenic S. suis is not a single uniform entity but comprises multiple lineages with distinct genome content and disease associations, raising the possibility that innate immune evasion mechanisms may not be equivalent across lineage. In the current article, we conducted a comprehensive review of the literature, focusing on the innate response developed mainly by phagocytes against this Gram-positive bacterium. Our aim was to summarize the last findings and to identify the main gaps in knowledge to guide future research efforts toward areas that require further investigation.