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Definition: a genus of gram-negative, facultatively anaerobic, rod-shaped bacteria of the family Enterobacteriaceae, consisting of nonmotile bacilli that cannot utilize citrate as a sole carbon source and that ferment carbohydrates with acid but no gas production. The genus consists of four species, differentiated by biochemical reactions: S. dysenteriae (subgroup A), S. flexneri (subgroup B), S. boydii (subgroup C), and S. sonnei (subgroup D). Their normal habitat is the intestinal tract of humans and higher monkeys; all species cause bacillary dysentery and shigellosis. Phylogenetically, Shigella is a pathovar of Escherichia coli, but because of long use its treatment as a separate genus has been preserved. The type species is S. dysente'riae
Source: Dorland's Illustrated Medical Dictionary. Philadelphia: Elsevier Health Sciences, 2007. Credo Reference. Web. 17 August 2012.