The cell wall of shigellae, like other Gram-negative bacilli, contains a lipopolysaccharide (LPS) structure. The LPS is liberated during lysis of the cell and, to some extent, during culture. The LPS moiety functions as an endotoxin and is an important component of the virulence of the bacteria.
The antigenic structure of shigellae is simple, unlike the com-plex antigenic structure of salmonellae. Shigellae possess somatic O antigens and certain strains possess K antigens. Shigella K antigens, when present, may sometimes interferewith agglutination by O antisera. Shigellae strains also possess fimbrial antigens. Common fimbrial antigens are also found particularly in S. flexneri.
S. flexneri is biochemically heterogeneous and antigenicallythe most complex among shigellae. S. flexneri, based on type-specific (I–VI) and group-specific (1–8) antigens, have been classified into six serotypes (1–6) and several subtypes (1a, 1b; 2a, 2b; 3a, 3b, 3c; 4a, 4b; 5a, 5b) (Table 33-3). Two antigenic variants, called X and Y, which lack the type specific antigens are also recognized in addition to this. S. flexneri serotype 6 is always indole negative and is classified into three biotypes: Boyd 88, Manchester, and Newcastle (Table 33-4).
S. sonnei is antigenically homogeneous but may occur in twoforms: phase I and phase II. Phase I strains produce smooth col-onies, while phase II colonies form large, flat, and more irregu-lar colonies. Cultures contain a mixture of both forms. Usually, phase II strains are isolated more frequently from convalescing cases and from carriers than from patients.