Shigella sonnei Shigella are Gram-negative, nonmotile, nonsporeforming rod-shaped bacteria. They are pathogens of humans and other primates, causing shigellosis. Depending on age and condition of the host, as few as 10 cells can be enough to cause an infection. The disease is caused when virulent Shigella organisms attach to, and penetrate, epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly, and spread to contiguous epitheleal cells resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin (very much like the verotoxin of E. coli O157:H7).