Shigella is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped and genetically closely related to E. coli. The genus is named after Kiyoshi Shiga, who first discovered it in 1897.The causative agent of human shigellosis, Shigella causes disease in primates, but not in other mammals. [Source: Wikipedia ]