Shigella is a gram-negative, nonmotile, non-lactose-fermenting aerobic bacillus, closely related to members of the genus Escherichia. The organisms are classified into four species or groups known as Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei (groups A, B, C, and D, respectively).
Members of groups A, B, and C exist in numerous serotypes, but only one serotype of group D is known. S. sonnei is the most commonly recovered Shigella species in the developed world, accounting for 70% of isolates in the United States. S. dysenteriae and S. flexneri are the most commonly recovered species of Shigella in the developing world.