» This article is about the bacteria. For the disease, see shigellosis Shigella is a genus of Gram-negative, non-spore forming rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella cause disease in primates, but not in other mammals. It is only naturally found in humans and apes. During infection, it typically causes dysentery. Classification Shigella species are classified by four serogroups: Serogroup A: S. dysenteriae (1… (More on Shigella)