Proteomic Analysis of Streptococcus suis During Exposure to Intracellular Condition of Human Macrophage U937 Cells

Streptococcus suis is an important zoonotic pathogen responsible for severe infections in pigs and humans. Its capacity to survive within phagocytic cells is considered a key virulence mechanism that contributes to dissemination and persistence in host tissues. This study employed comparative proteomic profiling to investigate intracellular adaptation of S. suis serotypes 2 (SS2) and 14 (SS14) during infection of human U937 macrophages. Five isolates originating from humans and pigs were analyzed using gel electrophoresis with liquid chromatography-tandem mass spectrometry (GeLC-MS/MS), revealing 118 differentially expressed proteins grouped into 11 functional categories. Translation-related proteins represented the largest group (48%), including upregulated ribosomal subunits (30S: S2, S5, S7, S8, S12, S15; 50S: L1, L5, L18, L22, L24, L33, L35) and translation factors such as GidA/TrmFO and RimP. Enrichment of carbohydrate metabolism and DNA replication proteins, including phosphoenolpyruvate carboxylase (PEP), UDP-N-acetylglucosamine pyrophosphorylase (GlmU), and ATP-dependent DNA helicase RuvB, indicated metabolic reprogramming and stress adaptation under intracellular conditions. Stress-response proteins such as molecular chaperone DnaK were also induced, supporting their multifunctional, "moonlighting" roles in virulence and host interaction. Comparative analysis showed that SS2 expressed a broader range of adaptive proteins than SS14, consistent with its higher virulence potential. These findings reveal conserved intracellular responses centered on translation, energy metabolism, and stress tolerance, which enable S. suis to survive within human macrophages. Integration of these intracellular proteomic signatures with previous exoproteomic, peptidomic, and network-based studies highlights translational and metabolic proteins-particularly DnaK, enolase, elongation factor EF-Tu, and GlmU-as multifunctional candidates linking survival and immunogenicity. This work establishes a comparative proteomic foundation for understanding S. suis intracellular adaptation and highlights potential targets for future vaccine or therapeutic development against this zoonotic pathogen.