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Shigella species cause shigellosis (bacillary dysentery), a human
intestinal disease that occurs most commonly among young children.
Shigellae are nonmotile, unencapsulated, and Lac-
. Most strains do not
produce gas in a mixed-acid fermentation of glucose
Shigella are typically spread from person to person, with contaminated stools serving as a major source of organisms. Humans are
the only natural host for Shigella species. Flies and contaminated
food or water can also transmit the disease. Shigellosis has a low
infectious dose: Approximately 10-100 viable organisms are sufficient to cause disease. Therefore, secondary cases within a house-hold are common, particularly under conditions of crowding or poor
sanitation. The 40 serotypes of Shigella are organized into four
groups (A, B, C, and D) based on the serologic relatedness of their
polysaccharide O antigens.
Shigella dysenteriae causes the
most serious infections, including HUS similar to that caused by
EHEC O157H7
S. dysenteriae type 1 produces Shiga toxin, which is structurally and genetically very similar to Shiga-like toxins 1 and 2 produced by E. coli virotypes. All Shiga and Shiga-like toxins are
capable of resulting in HUS in susceptible individuals.