Shigella is a genus of bacteria that is Gram-negative, facultatively anaerobic, non–spore-forming, nonmotile, rod-shaped, and is genetically closely related to Escherichia. The genus is named after Kiyoshi Shiga, who discovered it in 1897.

Gram-negative bacteria are bacteria that unlike gram-positive bacteria do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is their cell envelope, which consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. These bacteria are found in all environments that support life on Earth.