O-antigen polysaccharide from Shigella flexneri serotype 2a

The O-antigen polysaccharide from Shigella flexneri serotype 2a is a surface-exposed carbohydrate chain forming the repeating distal portion of lipopolysaccharide (LPS) on the bacterial outer membrane. Its structure consists of repeating pentasaccharide units with the backbone: \(\rightarrow 2)-\alpha-L-Rha III-(1 \rightarrow 2)-\alpha-L-Rha II-(1 \rightarrow 3)-\alpha-L-Rha I-(1 \rightarrow 3)-\beta-D-GlcNAc-(1 \rightarrow 4)-\beta-D-Glc\), variably substituted with glucosyl and acetyl groups, which define serotype-specific epitopes. The O-antigen is essential for the virulence of S. flexneri 2a, contributing to immune evasion, serum resistance, and modulation of membrane protein function. It is the principal antigenic target for vaccine development, as protective immunity is determined by the induction of antibodies recognizing this structure. Vaccines such as Sf2a-EPA are based on this antigen and have demonstrated immunogenicity and protective efficacy in preclinical and clinical studies. The O-antigen acts as the major determinant of serotype, is a critical molecular fingerprint for diagnostics, and is not a protein, enzyme, or receptor itself, but a key surface antigen exploited as a vaccine target.

Other names

Shigella flexneri serotype 2a O-polysaccharideS. flexneri 2a O-antigenS. flexneri 2a O-polysaccharideO-antigen from S. flexneri 2a

Conjugate vaccines induce protective antibody response against O-antigen. Antibody binding leads to complement-mediated bacterial killing. No enzymatic/receptor inhibition occurs, mechanism is via immune recognition.