Shigella is the most common cause of bacillary dysentery, whichoccurs worldwide. The disease is spread through fecal–oral transmission, and humans are the only natural reservoir of the bacteria. Human infections caused by Shigella species are summarized in Table 33-1.
Based on a combination of biochemical and serological characteristics, shigellae are classified into four species or subgroups, consisting of more than 45 O antigen-based serogroups and each species consisting of different serotypes.
Shigella dysenteriae (group A):This is subdivided into 12serotypes. Each serotype is characterized by the presence of a different type of antigen. S. dysenteriae serotype 1 is the bacillus originally described by Shiga, hence known as Shiga’s bacillus. S. dysenteriaeserotypes 3–7 were described by Large and Sachs inIndia, and hence were known as the Large–Sachs group.
Shigella flexneri (group B):This group is named after Flexner(1900), who described the first of the mannitol-fermenting shigellae from Philippines. 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).
Shigella boydii (group C):This group is named after Boyd,who first described these strains from India (1931). A total of 19 serotypes have been identified in this group. Members of S. boydii group resemble biochemically, but not antigenically,with those of S. flexneri.
Shigella sonnei (group D):This group is named after Sonne, whofirst described these strains from Denmark (1915). S. sonnei have been divided into 26 colicin types by using 33 indicator strains.