Salmonella Paratyphi A O:2 O-polysaccharide (SPA O:2 OPS)

Salmonella Paratyphi A O:2 O-polysaccharide is a key surface antigen of the Gram-negative bacterium Salmonella enterica serovar Paratyphi A, the primary cause of paratyphoid fever (NIH, 2012). It is the outermost part of the lipopolysaccharide (LPS) molecule and consists of a repeating trisaccharide backbone (rhamnose, mannose, and galactose) with a characteristic paratose branch that defines the O:2 serogroup specificity (NIH, 2025). This polysaccharide serves as a critical virulence factor, protecting the bacterium from host immune defenses while also acting as a primary target for the host's humoral immune response (ResearchGate, 2024). In clinical development, it is the central component of conjugate vaccines, where it is chemically linked to carrier proteins like CRM197 or tetanus toxoid to enhance immunogenicity, particularly in young children (NIH, 2012; NIH, 2025). These vaccines aim to elicit high titers of bactericidal antibodies that facilitate the clearance of the pathogen through complement-mediated killing or opsonophagocytosis (Nature Medicine, 2025). The structural integrity of the polysaccharide, including its O-acetylation and glucosylation patterns, is vital for maintaining its protective epitopes and ensuring vaccine efficacy (NIH, 2020).

Other names

Salmonella Paratyphi A O-antigenSalmonella Paratyphi A O-specific polysaccharideSPA OSPSPA OAgSalmonella Paratyphi A lipopolysaccharide

Induction of active immunity through the production of bactericidal antibodies (IgG and IgA) that recognize the O-antigen, leading to complement-mediated lysis or opsonophagocytosis of the bacteria.