Shigella serogrouping relies on which surface antigen?

• A. Vi antigen
• B. H antigens
• C. K antigens
• D. O antigens

Answer: (D)

Shigella serogrouping is determined by the O antigen, the somatic polysaccharide part of the outer membrane’s lipopolysaccharide. Variations in these O antigens create distinct serogroups, and typing with antisera against specific O antigens yields the characteristic agglutination patterns used to classify Shigella. In contrast, Vi is a capsule antigen found in Salmonella Typhi, H antigens are flagellar proteins (and Shigella are typically nonmotile, so these aren’t used for its serogrouping), and K antigens are capsule determinants in other Enterobacteriaceae but not the basis for Shigella serogroups. Hence, the defining surface antigen for Shigella serogrouping is the O antigen.