The method of serogrouping Shigella used in the clinical laboratory is based on which antigens?

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

Answer: (D)

Shigella serogrouping is determined by the somatic O antigens of the bacterial outer membrane lipopolysaccharide. These O antigens vary among the Shigella serogroups and are detected with specific antisera in agglutination tests, allowing classification into the major groups. The Vi antigen is a capsule found in Salmonella Typhi, not Shigella. H antigens are the flagellar antigens used for motile bacteria; Shigella is nonmotile and lacks H antigens, so they aren’t used for serogrouping. K antigens refer to capsular polysaccharides in some bacteria, but Shigella serogrouping relies on O antigens. So, the method of serogrouping Shigella is based on O antigens.