genus of rod-shaped bacteria in the family Enterobacteriaceae, species of which are normal inhabitants of the human intestinal tract and can cause dysentery, or shigellosis. Shigella are microbiologically characterized as gram-negative, non-spore-forming, nonmotile bacteria. Their cells are 0.4 to 0.6 micrometre across by 1 to 3 micrometres long. S. dysenteriae, spread by contaminated water and food, causes the most severe disease because of its potent exotoxin, but S. sonnei and S. flexneri are also implicated as dysentery agents.