Shigellosis is caused by Shigella dysenteriae, a gram-negative, non-motile bacterium responsible for bacillary dysentery. It invades the colonic mucosa, leading to inflammation, ulceration, and bloody diarrhea. The organism produces the Shiga toxin, which contributes to mucosal damage and systemic complications. This pathogen's ability to penetrate epithelial cells distinguishes it from other enteric bacteria such as Salmonella, E. coli, and Vibrio cholerae. Therefore, Shigella dysenteriae is the definitive causative agent of shigellosis.

Reference: Medical Microbiology, Murray et al., 9th Edition.