Shigella spp.
Shigella spp. (S. dysenteriae, Shigella flexneri, Shigella boydii, and 
Shigella sonnei) are nonmotile; lysine decarboxylase–negative; 
citrate-, malonate-, and H2
S-negative; non–lactose fermenting; 
gram-negative rods that grow well on MacConkey agar. All 
strains ferment glucose without gas production, except a few 
strains (S. flexneri serotype 6 and S. boydii serotype 14). The 
four subgroups of Shigella spp. are S. dysenteriae (group A), 
S. flexneri (group B), S. boydii (group C), and S. sonnei (group 
D). Each subgroup has several serotypes. Serotyping is based on 
the somatic LPS O antigen. Shigella and inactive E. coli strains 
are often very difficult to distinguish. After presumptive identi fication of a suspected Shigella species based on traditional bio chemical methods, serotyping should be completed, especially 
in the case of S. dysenteriae. Suspected strains of Shigella spp. 
that cannot be typed by serologic methods should be referred 
to a reference laboratory for further testing.
Shigellosis may begin as watery diarrhea, fever, and 
abdominal cramps. Progressive infection then leads to dys entery with stools that contain blood or mucus. Lesions in 
the intestinal tract typically remain confined to the large 
intestine. Bloodstream infections are rare. All Shigella
are capable of causing dysentery. However, S. dysenteriae
1 produces the most severe forms of illness and may lead to 
hemolytic uremic syndrome (HUS).