Salmonella Typhimurium O-specific polysaccharide (O-Ag)

The Salmonella Typhimurium O-specific polysaccharide, also known as the O-antigen, is a repeating carbohydrate polymer that constitutes the outermost part of the lipopolysaccharide (LPS) on the bacterial surface. It serves as a major virulence factor by providing a protective barrier that enables the bacterium to evade the host's innate immune system, specifically by conferring resistance to complement-mediated killing and reducing opsonophagocytosis. The structure is characterized by specific repeating units (typically a tetrasaccharide backbone including abequose, mannose, rhamnose, and galactose) that define the serotype identity, such as the O4, O5, and O12 antigens characteristic of serovar Typhimurium. Beyond immune shielding, the O-specific polysaccharide is critical for environmental persistence, biofilm formation, and successful colonization of the host intestinal tract. In therapeutic development, this molecule is a high-priority target for the prevention of salmonellosis and invasive non-typhoidal Salmonella (iNTS) disease. Due to its poor immunogenicity as a standalone T-cell independent antigen, it is frequently conjugated to carrier proteins—such as tetanus toxoid or flagellin—to form conjugate vaccines that elicit robust and long-lasting antibody responses. These vaccines and experimental monoclonal antibodies aim to induce opsonizing and bactericidal antibodies that neutralize the pathogen and facilitate its clearance. Furthermore, the O-antigen serves as the primary receptor for many bacteriophages, making it a central focus for phage-based therapeutic and diagnostic strategies.

Other names

O-antigenO-AgS. Typhimurium O-polysaccharideSalmonella Typhimurium lipopolysaccharide O-side chainGroup B O-antigenSalmonella Typhimurium O-specific antigenOPSCOPS (Core-plus-O-polysaccharide)